1570s, "a deviation in argument," also "a military sally," from Latin excursionem (nominative excursio) "a running forth, sally, excursion, expedition," figuratively "an outset, opening," noun of action from past participle stem of excurrere "run out, run forth, hasten forward; project, extend," from ex- "out" (see ex-) + currere "to run" (see current, adj.). Sense of "journey" recorded in English by 1660s.