rioter n.

c. 1200, "the following of a wrong scent by hounds" (a sense now obsolete but in one phrase); early 14c., "debauchery, extravagance, wanton living," from Anglo-French rioute, Old French riot, riote (12c.) "dispute, quarrel, (tedious) talk, chattering, argument, domestic strife," also a euphemism for "sexual intercourse," of uncertain origin. Compare Italian riotta (Medieval Latin riota) "quarrel, dispute, uproar, riot." Perhaps from Latin rugire "to roar."

The meaning "civil disorder, violent disturbance of the peace, public disturbance arising from wanton and disorderly conduct" is attested by late 14c. The meaning "something spectacularly successful" first recorded 1909 in theater slang. The sense of "vivid display of colors" is by 1891.

To run riot "act or move without control or restraint" is by 1520s, a figurative extension of the oldest Middle English meaning of the word, in reference to hounds following the wrong scent. The Riot Act, part of which must be read to a mob before active measures can be taken, was passed 1714 (1 Geo. I, st.2, c.5). Riot girl and alternative form riot grrl first recorded 1992.

English agent noun ending, corresponding to Latin -or. In native words it represents Old English -ere (Old Northumbrian also -are) "man who has to do with," from Proto-Germanic *-ari (cognates: German -er, Swedish -are, Danish -ere), from Proto-Germanic *-arjoz. Some believe this root is identical with, and perhaps a borrowing of, Latin -arius (see -ary).

Generally used with native Germanic words. In words of Latin origin, verbs derived from past participle stems of Latin ones (including most verbs in -ate) usually take the Latin ending -or, as do Latin verbs that passed through French (such as governor); but there are many exceptions (eraser, laborer, promoter, deserter; sailor, bachelor), some of which were conformed from Latin to English in late Middle English.

The use of -or and -ee in legal language (such as lessor/lessee) to distinguish actors and recipients of action has given the -or ending a tinge of professionalism, and this makes it useful in doubling words that have a professional and a non-professional sense (such as advisor/adviser, conductor/conducter, incubator/incubater, elevator/elevater).