'inexcitable' originates from the prefix 'in-' (a negating prefix) combined with 'excitable', which ultimately comes from Latin 'excitabilis' via Old French and Middle English; 'excitare' in Latin meant 'to rouse or call out'.

'inexcitable' was formed by adding the negative prefix 'in-' to 'excitable'. 'Excitable' comes from Latin 'excitabilis' (from 'excitare'), passed into Old French as 'exciter' and into Middle English as 'exciten'/'excite', later yielding 'excitable' and then 'inexcitable' in Modern English.

Originally the Latin root 'excitare' meant 'to rouse or summon forth'; over time the sense crystallized around 'to stir up emotion or activity', and 'inexcitable' developed the current meaning 'not easily stirred or excited'.