The style and attitude of punk rock--mohawks, anarchy and leather jackets--are what most people think of when they use the word "punk." But back in 16th century England, a "punk" was a prostitute.
By the 1920s, "punk" had developed into a slur for men who were the willing or unwilling submissive sex partners of other men in jails. As punks tended to be on the younger side, the word's meaning morphed into "juvenile delinquent." When the '50s rolled around, people hurled the term at young, leather-jacketed street toughs and hot-rodders.
In the 1970s, "punk" was co-opted by two young New York fanzine creators who welcomed the notion of being considered delinquents. They wrote about the new music that was emerging in reaction to the monster rock that ruled the airwaves, and named their zine "Punk." The music--from bands like the Ramones, Blondie and the diverse set of "street rockers" that coalesced around New York's CBGB--came to be known as punk, too.
From New York, "punk" music spread to England, where it became associated with the politics of anarchy and developed an aggressive aesthetic sensibility, thanks in no small part to the Sex Pistols. Punk became a revolt against conventional life and politics as well as music. This is where the classic punk look developed--safety pins, torn clothing and dyed, spiked hair--as an in-your-face attack on the mainstream.
These days, "punk" refers to a look, an attitude, an ethos, a style of music or a person who seems to espouse any of these. It can be hurled as an insult to label someone as degenerate or antisocial, or it can be worn as a badge of pride. Beyond that, however, the definition of punk has become confused (it can be a pretty touchy subject) because of its commercialization and the development of so many kinds of punk music and communities: racist skinheads, non-racist straight-edge skinheads and crusty punks, just to name a few.