Mr. Webster provides a no-nonsense and old-fashioned definition of this word: for him and for his nineteenth-century readers, being on the defensive is simply the opposite of being on the offensive. If you're aiming your ballista at someone, you're on the offensive; if you're building a wall, you're on the defensive.
That's all well and good. But these days the word "defensive" is often used in a much more abstract way, to describe a particular psychological state.
"Don't get defensive!", Or, The State Of Mind Of The Lady Who Doth Protest Too Much
The popular idea of defensiveness is a descendent of Sigmund Freud's concept of "reaction formation" (Reaktionsbildung), which is one of his classic defense mechanisms (Abwehrmechanismus).
For Freud, the Ego, which is to say the day-to-day self, is constantly caught between the violent and lustful desires of the "animal" side of the psyche (usually translated into English as the "Id", though I prefer the literal translation, the "It"), on the one hand, and the controlling, shaming, and punishing inner censor (which is usually called the Superego, literally the "Over-Me"), on the other.
When tensions arise between the Id's passions and the Superego's insistence on propriety, a person feels anxiety. The Ego uses various strategies to ease this anxiety and to make ordinary life more tolerable. The problem is that many of these strategies involve lying to oneself, finding a scapegoat, and treating very real problems as if they don't exist. For Freud, then, the goal of psychoanalysis is to reveal the hidden desires that are obscured by the ego's defense mechanisms; to understand the fears and drives that give rise to them; and to address those fears and drives directly rather than through inefficient neurotic behaviours.
Reaction formation takes place when a person takes a loud, public stand against their own position. At first this may sound counter-intuitive -- who would do that? -- but a quick look at the news makes it quite clear that reaction formation is all over the place. When the chairman of the U.S. House Caucus on Missing and Exploited Children is discovered to be writing racy instant messages to adolescent pages; when a prominent evangelical Christian preaches that homosexuality is a sin and an aberration but is then exposed to have hired male prostitutes; when the same man who tried to remove the words "sexual orientation" from U.S. hate crimes bills was caught soliticing sex from an undercover cop in an airport bathroom; when a New York governor who rails against government corruption is involved with a massive prostitution and money-laundering ring... it's very, very hard to avoid the conclusion that Freud was right.
Now when I say Freud was right about this, I do not mean that all homophobes are secretly gay. Rather, I am saying that homophobes have anxieties about homosexuality, and those anxieties may include latent homosexual desires. They may not. Similarly, the knight who appoints himself defender of public morals may honestly believe that the work he is doing is good. Freud didn't accuse neurotics of being evil or malicious; rather, he accused them of being deluded. Caught between warring drives, and without a clear-eyed view of their own desires, they become more and more mentally divided, often to the point of leading double lives (or at the very least being hypocrites).
At any rate, a furious attack on a position, for Freud, is a sign that the person on the attack is afraid of her own attraction to that same position. Thus, a person struggling with alcoholism may be an outspoken critic of the demon drink, angrily chastizing anyone who gets drunk in her presence. Or a man who is afraid that he is falling in love with a woman starts being nasty to her, humiliating her in front of other people, or dwelling obsessively on all the things about her that drive him crazy.
All this talk about attacks! So why is it called "defensiveness," then?
Defensiveness is not quite the same thing as reaction formation. Reaction formation is indeed "offensive," in that a person actively constructs a position opposite to the one she actually holds (or fears she might hold, or would hold if she were courageous enough to face the consequences of holding it) and manifests that false position in the world.
Defensiveness, by contrast, is a hasty denial of a position when confronted with it. No doubt everyone reading this node has experienced a conversation along the lines of the following:
"Hey, that guy at the other end of the bar is pretty cute, don't you think?"
"Bullshit! He's ugly as sin!"
"But he seems like just your ty--"
"Besides, even if I
did think he was cute, I am totally in love with my current boyfriend and I have no interest in looking for someone else!"
"I wasn't suggesting that you were. I was just saying--"
"My boyfriend and I are getting along fantastically, and the sex is great, thank you very much!"
"I really didn't need to kn--"
"But even if we weren't, and even if it wasn't, I am totally not into that guy at the end of the bar!"
"I think you're getting a bit defensive."
"I AM NOT."
The idea of defensiveness is an extremely useful way to talk about the hidden motivations of people. However, the problem with it (and with so many other Freudian concepts dependent on the existence of the unconscious), is that there is really no... well, defense against an accusation of it. Consider this variation of the previous dialogue.
"Hey, that guy at the other end of the bar is pretty cute, don't you think?"
"Eh. Not my type."
"Don't be so defensive!"
"What? I'm just not into him."
"Suuuuuure you aren't! I know what you're really thinking! You wanna hop in the sack with him right now!"
"Er, no."
"Man, you are SO OBSESSED."
The second dialogue is a classic case of what Freud would call projection; the first speaker is clearly the one who is obsessed, since she simply won't let the subject go. Like that speaker, a bad psychoanalyst (a cynic would say all psychoanalysts) can easily fall into the trap of finding "hidden motivations" where there aren't any. If the analyst isn't careful to distinguish honest denials from neurotic ones, then the patient can be diagnosed with just about anything.
The term "defensive" has entered the English vernacular in a way that "reaction formation" has not, and consumers of the mass media have been trained to recognize it in numberless sitcoms. The next time you hear someone screeching "I am not!", it's worth considering just why we don't take them at their word.