From Wiktionary:

• A person who is lame due to a crippling of the legs or feet.

• (slang, derogatory) A name-calling word, generally for a person who is perceived to be inept, deficient or peculiar.

• (BDSM) A submissive dressed in a full-body suit of latex or a similar material.

I'm vaguely sure the first two meanings above appeared earlier. Perhaps the name was taken as a nod to ‘geek’ and ‘dweeb’, which were slurs for programmers back before the profession became mainstream. Edit: apparently not: "The software was originally named the General Image Manipulation Program. Kimball and Mattis formed the acronym GIMP by adding the letter G to '-IMP,' inspired by a reference to 'the gimp' in the 1994 film Pulp Fiction."

Somewhat relatedly, Torvalds called his versioning software ‘git’, which is also a derogatory term, a British-specific one at that. (And the original name of Linux was 'Freax'.)

ENGLISH, MOTHERFUCKER! DO YOU SPEAK IT?

Graphics Image Manipulation Program. That what GIMP means.