'noncaustic' originates from English, formed by the prefix 'non-' (from Latin 'non' meaning 'not') attached to 'caustic', which ultimately derives from Greek 'kaustikos' (from 'kaiein' meaning 'to burn').

'caustic' entered English via Late Latin 'causticus' and French 'caustique', from Greek 'kaustikos'; the Modern English adjective 'noncaustic' was created by adding the productive negative prefix 'non-' to 'caustic'.

Initially, 'caustic' meant 'capable of burning' (literally); over time it also developed a figurative sense of 'biting' or 'sarcastic'. 'Noncaustic' therefore denotes both the literal absence of corrosive/burning quality and, by extension, the absence of a harsh or sarcastic tone.