'irritably' originates from the Latin word 'irritabilis', where 'irritare' meant 'to provoke or annoy'.

'irritabilis' transformed into the Old French word 'irritable', and eventually became the modern English word 'irritable', with 'irritably' as its adverbial form.

Initially, it meant 'easily provoked or annoyed', and this meaning has largely remained the same in modern usage.