'few-nerved' is an English compound formed from 'few' + past-participle/adjectival use of 'nerve' (from the noun 'nerve'). 'few' comes from Old English 'fēawe' meaning 'not many', and 'nerve' ultimately from Latin 'nervus' via Old French 'nerf'.

'nerve' entered Middle English from Old French 'nerf', which came from Latin 'nervus'. The compound adjective 'few-nerved' arose in English by combining 'few' with the adjectival sense of 'nerved' (literally 'having few nerves') and was used figuratively to mean lacking courage.

Originally the components could suggest a literal idea of 'having few nerves' (physiological), but over time the compound came to mean 'lacking courage or nerve' (figurative), and that is the predominant modern sense.