Stirred up into a state of anger or agitation. It describes being provoked or disturbed from a calm state into irritation or upset.
Possibly from 'roil' meaning to disturb or make turbid, related to Old French 'rouiller' (to rust, make muddy). The sense of stirring up emotions like sediment in water appeared in American English around the 19th century.
The water metaphor in 'riled' is psychologically perfect - calm emotions can be stirred up just like sediment in a clear pond. This imagery captures how external provocations can cloud our usually clear thinking and create emotional turbulence that takes time to settle back down.
Complete word intelligence in one call. Free tier — 50 lookups/day.