'gnatlike' originates from English, specifically from the noun 'gnat' and the suffix '-like'; 'gnat' itself goes back to Old English 'gnæt' (from Proto-Germanic *gnatuz), where 'gnat' meant 'a small biting fly', and the suffix '-like' derives from Old English 'lic' meaning 'having the form of' or 'like'.

'gnat' changed from Old English 'gnæt' (and related Proto-Germanic forms) into Middle English forms such as 'gnat' or 'gnatte', while the adjectival suffix came from Old English 'lic' → Middle English '-liche' and eventually became the modern English suffix '-like', combining to form 'gnatlike' in Modern English.

Initially it meant 'resembling a gnat (in form or appearance)'; over time it retained that literal sense but also developed figurative uses meaning 'very small' or 'annoyingly petty/persistent'.