'wood-gnat-like' originates from Modern English, specifically the elements 'wood', 'gnat', and the suffix '-like', where 'wood' meant 'tree' or 'forest area', 'gnat' meant 'a small flying insect', and '-like' meant 'having the nature of'.

'wood-gnat-like' is a modern compound formed by combining Old English-derived words: 'wood' (from Old English 'wudu'), 'gnat' (from Old English 'gnæt' or 'gnætt', of Germanic origin), and the adjectival suffix '-like' (from Old English 'līc/gelīc' meaning 'similar'). Over time these elements were freely combined in Modern English to form descriptive compounds like this one.

Initially the components meant 'tree/forest' + 'small fly' + 'similar to'; the compound's sense of 'resembling a wood gnat' follows directly from those elements and has no major semantic shift in modern usage.