"one who forsakes cause, duty, party, or friends," 1630s, agent noun from desert (v.). Especially "soldier or sailor who departs from position without leave and without intent to return" (1660s).
Entries linking to deserter
c. 1600, transitive, "to leave, abandon," either in a good or bad sense; 1640s, in reference to military service or duty, "leave without permission;" from French déserter "cause to leave," literally "undo or sever connection," from Late Latin desertare, frequentative of Latin deserere "to abandon, to leave, forsake, give up, leave in the lurch," from de "undo" (see de-) + serere "join together, put in a row" (from PIE root *ser- (2) "to line up"). Intransitive sense of "quit (a service or post) without permission" is from 1680s. Related: Deserted; deserting.
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).