the term side is an odd one, but it comes from the already english terms of top and bottom. so instead of being a top or a bottom, you're on the side. because you don't do anal, so you're like, side to side with your partner, masturbating, oral as well. basically everything except anal play.
doesn't really work outside of english, because english is the only language, afaik, that uses top and bottom as terms for sexual positions. other languages call tops "actives" and bottoms "passives"(in the respective languages ofc). so a side in non english languages would what, neutral?