'off-mainstream' originates from English, combining the preposition 'off' (from Old English 'of', meaning 'away') and the compound 'mainstream' (formed from 'main' + 'stream', where 'main' meant 'chief' and 'stream' meant 'current').

'mainstream' developed in the early 20th century from the literal sense 'main + stream' (principal current) to the figurative sense 'dominant trend'. The phrase 'off the mainstream' was later shortened and reanalysed into the compound 'off-mainstream' to describe things away from that dominant trend.

Initially, the components referred to physical direction or current ('off' = away; 'mainstream' = principal current). Over time 'mainstream' came to mean 'dominant or conventional trend', and 'off-mainstream' evolved to mean 'away from or outside that conventional trend'.