'postmarker' originates from Modern English, formed from the prefix 'post-' and the word 'marker'; 'post-' ultimately originates from Latin, specifically the word 'post', where 'post' meant 'after', and 'marker' ultimately originates from Old English, specifically the word 'mearc/mark', where 'mearc/mark' meant 'sign'.

'postmarker' developed as a compound from the earlier phrase 'post marker' (two words) and from the productive English prefix 'post-' (adopted via Medieval Latin/Old French from Latin 'post') combined with 'marker' (from Old English 'mearc/mark'), eventually stabilizing as the single lexical item 'postmarker' in modern linguistic usage.

Initially it literally meant 'a marker placed after (something)', and over time the term became specialized in linguistics to mean 'a grammatical marker placed after a word' (often synonymous with or analogous to a postposition).