Etymology of "punk"?

The word existed with a different meaning back in the times of Shakespeare

From Wikipedia

From the late 16th through the 18th century, punk was a common, coarse synonym for prostitute; William Shakespeare used it with that meaning in The Merry Wives of Windsor (1602) and Measure for Measure (1623). The term eventually came to describe "a young male hustler, a gangster, a hoodlum, or a ruffian".

As Legs McNeil explains, "On TV, if you watched cop shows, Kojak, Baretta, when the cops finally catch the mass murderer, they'd say, 'you dirty Punk.' It was what your teachers would call you. It meant that you were the lowest." The first known use of the phrase punk rock appeared in the Chicago Tribune on March 22, 1970, attributed to Ed Sanders, cofounder of New York's anarcho-prankster band The Fugs. Sanders was quoted describing a solo album of his as "punk rock—redneck sentimentality". In the December 1970 issue of Creem, Lester Bangs, mocking more mainstream rock musicians, ironically referred to Iggy Pop as "that Stooge punk". Suicide's Alan Vega credits this usage with inspiring his duo to bill its gigs as a "punk mass" for the next couple of years.

Etymology Online also confirms

"worthless person" (especially a young hoodlum), 1917, probably from punk kid "criminal's apprentice," underworld slang first attested 1904 (with overtones of "catamite"). Ultimately from punk "prostitute, harlot, strumpet," first recorded 1590s, of unknown origin. For sense shift from "harlot" to "homosexual," cf. gay. By 1923 used generally for "young boy, inexperienced person" (originally in show business, e.g. punk day, circus slang from 1930, "day when children are admitted free"). The verb meaning "to back out of" is from 1920. The "young criminal" sense is no doubt the inspiration in punk rock first attested 1971 (in a Dave Marsh article in "Creem"), popularized 1976.

Related to the noun punk, that is decayed wood which is of inferior or nearly worthless quality for the common uses of wood like heating fuel or building material. Certainly related to the denigration of people who are seen as worthless or of low "quality" like prostitutes or young men of low morals.

Growing up in rural Maine in the 1970s with my grandfather who was born in the early 20th century, we used the noun punk or the adjective punky to refer to wood we rejected as we gathered fuel for heating our home. When I first heard punk used to describe a young man, I associated it with the worthlessness of rotten wood, and thus a person of little worth.