'uncontentious' is formed from the negative prefix 'un-' (Old English 'un-' meaning 'not') attached to 'contentious', which ultimately comes from Latin 'contentiosus' (from 'contentio'/'contentio-', related to 'contendere').

'contentious' came into English via Old French (e.g. 'contentieux') and Middle English as 'contentious'; the negative prefix 'un-' (Old English) was later attached to create 'uncontentious' meaning 'not contentious'.

Originally, 'contentious' meant 'causing contention, inclined to dispute' (from Latin roots meaning 'to strive/contend'); 'uncontentious' has retained the straightforward negative sense 'not causing contention' and is used to describe things unlikely to provoke argument.