A daemon / demon was a procedure which runs when a slot is accessed / added / deleted / .... My bet is that this name was used in frame languages in the end 70s

A daemon / demon was a procedure which runs when a slot is accessed / added / deleted / .... My bet is that this name was used in frame languages in the end 70s where there were if-needed, if-added, ... demons for a frame slot. Flavors probably got it from there - though I'm not sure. But the concept/term of a demon goes back to the end 60s to Hewitt's planner language and probably even earlier. Later with generic functions in CLOS, I'd guess the term wasn't used, since something like an :after method is no longer conceptually a specific demon procedure attached to a slot via a facet, but a method in a method combination.