in computer sense, as one word, by 1975, from log (v.2) + on (adv.).

word-forming element meaning "one who is immersed in or driven by," mostly from French-derived words, ultimately from Greek -logos, -logon (see -logy). Now mostly superseded by -loger, -logist except in ideologue and a few others. As a combining element meaning "kind of discourse," it is from French -logue, from Greek -logos.