'single-tone' is a compound formed from English 'single' and 'tone'. 'single' originates from Latin, specifically the word 'singulus', where the root meant 'one, individual'. 'tone' originates from Greek, specifically the word 'tonos', where it meant 'stretching, tension' and later 'pitch, sound'.

'singulus' became Old French 'sengle' / 'sengle' and then Middle English 'single'; 'tonos' passed into Latin as 'tonus' and via Old French/Middle English became 'tone'. These elements were combined in modern English to form the compound 'single-tone'.

Originally 'singulus' meant 'one' and 'tonos' referred to 'stretch/tension' (and later musical pitch). Over time their combination came to mean 'a single pitch or sound' in modern usage as 'single-tone'.