c.1200, "chance, a person's luck, fortune, fate;" also "unforeseen occurrence," from Old Norse happ "chance, good luck," from Proto-Germanic *khapan (source of Old English gehæp "convenient, fit"), from PIE *kob- "to suit, fit, succeed" (cf. Old Church Slavonic kobu "fate, foreboding, omen," Old Irish cob "victory"). Meaning "good fortune" is from early 13c.
"to happen," mid-14c., from hap (n.) "chance."