SKILL is not an acronym, just a name (which should always be capitilized for Trademark reasons). However, early on there was SCIL which was something like Silicon Compiler Interface Language, which was pronounced "skill" and probably is the origin of the name we have today.
SKILL was originally based on a flavor of Lisp called "Franz Lisp" which was eventually superseded by an ANSI standard called Common Lisp. From here the Common Lisp Object System (CLOS) was developed, and ideas from here as well as from Scheme were taken for the development of SKILL++ (object oriented SKILL).
Hope this answers your questions.