From Middle English hap, happe, from Old Norse happ, from Proto-Germanic *hampą, from Proto-Indo-European *kob-. Cognate with Icelandic happ. Related also to Icelandic heppinn, Old Danish hap, Old English ġehæp, Swedish hampa, Old Church Slavonic кобь, Old Irish cob.
The verb is from Middle English happen, from Old Norse *happa, *heppa, from Proto-Germanic *hampijaną ("to fit in, be fitting"), from the noun. Cognate with Old Danish happe ("to chance, happen"), Norwegian heppa ("to occur, happen").