'casually-hired' originates from English, specifically the words 'casual' and 'hire', where 'casual' came via Old French from Latin 'casualis' meaning 'by chance' and 'hire' came from Old English 'hȳrian' meaning 'to employ'.

'casual' passed into English from Old French/Latin with senses of 'by chance' and later 'not regular', while 'hire' has Old English roots meaning 'to employ'; the compound/phrase 'casually-hired' developed in modern English to describe employment of a non-regular, temporary kind.

Initially, 'casual' meant 'by chance' (and 'hire' meant 'to employ'); over time 'casual' shifted toward 'not regular/temporary', and 'casually-hired' has come to mean 'employed temporarily or without firm commitment'.