Also a slang synonym for human female breasts, almost always used exclusively in the plural, and usually considered vulgar. The Oxford English Dictionary earliest citation in this sense for "knocker" is from 1941 by the obscure Jack Smiley in Hash House Lingo and reads "Fix the knockers — look at the nice breasts on that woman". Lest the reputability of the term be cast in doubt, the next citation is from Norman Mailer's Naked and Dead in 1949: "Look at the knockers on her, Murray says." It is almost undoubtedly then a very recent addition to the English lexicon, from the 20th century in this meaning.
The etymology isn't completely straightforward, but I surmise that it must be at least related to the much older meaning of "knocker" as a "looker", i.e. a person, usually a woman, whose beauty is so striking that it actually knocks you. In this sense the OED lists citations as far back as the 1600's. Certainly there have been known more than one set of human female breasts that have had a knockout effect on the opposite sex.