'unaroused' is formed in English by adding the negative prefix 'un-' to 'aroused' (the past participle of 'arouse'); 'un-' comes from Old English 'un-' meaning 'not', and 'arouse' comes from Middle English < 'arousen' from Old French 'arouser', where the element 'a-' is a verb-forming prefix and 'rouse' meant 'to stir up'.

'arouse' came into Middle English as 'arousen' from Old French 'arouser' (based on a- + rouse), then English formed the participle 'aroused'; later the productive English prefix 'un-' was attached to yield 'unaroused' meaning 'not aroused'.

The components originally referred to 'stirring up' or 'raising' (arouse) and the negative prefix 'un-' meant 'not'; combined they have kept the straightforward negative sense 'not aroused' in modern use.