'single-word' is a modern compound made from 'single' + 'word'. 'single' ultimately comes from Latin 'singulus' (meaning 'one each' or 'single') via Old French and Middle English; 'word' comes from Old English 'word', from Proto-Germanic (cf. German 'Wort'), meaning 'speech' or 'unit of language'.

'single' developed from Latin 'singulus' into Old French and Middle English forms (e.g. Middle English 'sengle'/'single'), eventually becoming modern English 'single'. 'word' has remained relatively stable from Old English 'word' to modern English 'word'. The compound 'single-word' is a straightforward modern English compound combining these two elements.

Individually, 'single' originally meant 'one, individual', and 'word' meant 'speech unit'; the compound retained the combined literal meaning 'one word' and has been used adjectivally (e.g. 'single-word answer') and nominally with little semantic shift.