A girl gun is a firearm, sold or shown to a member of the fairer sex
by a macho
guy, that is not something that the guy himself would carry into a dark alley. The basis is that the girl can't handle a bigger gun, and any girl who buys such a firearm is asking for trouble, because they frequently aren't powerful enough to actually stop an attacker.
.17 caliber, .22 caliber, .25 caliber, and .27 caliber handguns are a prime example of girl guns, when they are offered as a primary means of defence. They are perfectly legitimate firearms to offer someone who is looking for an easily concealable holdout
gun. That is, they are much better than nothing, in that they might distract your attacker long enough for you to run away, or you might get lucky
, but you really ought to have something bigger if you really want to defend yourself.
These are also perfectly legitimate rifle calibers. But a handgun doesn't have enough space to develop enough power behind these rounds to be effective.
A .22 caliber machine pistol is not a girl gun. But it's also really really illegal to carry one concealed, and you're not likely to find one in most local gun shops. (Unless you live in Las Vegas.)