'gap-billed' originates from Modern English, specifically the compound of 'gap' + 'bill' with the suffix '-ed', where 'gap' meant 'an opening' and 'bill' meant 'beak'.

'gap' traces back to Old Norse/Old English words meaning 'opening' (Old Norse 'gap', Old English forms such as 'geap'); 'bill' comes from Old English/Middle English forms (e.g. Old English 'bile', Middle English 'bille') meaning 'beak'; the adjectival use with '-ed' developed through English word-formation to create compounds like 'gap-billed'.

Initially the elements referred separately to 'an opening' and 'a beak', but over time the compound came to mean 'having a gap or notch in the bill' when applied to birds.