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" (from PIE root *kers- "to run"). Sense of "journey" recorded in English by 1660s.
