Actually "LL" or SKILL isn't directly an acronym. This is from an old internal web page describing the history of SKILL:
'SKILL was originally based on a flavor of Lisp called "Franz Lisp." Franz Lisp and all other flavors of Lisp were eventually superseded by an ANSI standard for Lisp called "Common Lisp." Historically, SKILL was known as IL. SKILL was a library of IL functions. The name was originally an initialism for Silicon Compiler Interface Language (SCIL), pronounced "SKIL," which then morphed into "SKILL," a plain English word that was easier for everyone to remember.'
From memory "IL" was just Interface Language. Whilst SKILL was used initially to describe the API rather than the language, the snappier name stuck. The name "IL" remains in the de-facto file extension used for SKILL code ".il".