happenyoudaoicibaDictYouDict[happen 词源字典]
happen: [14] Surprisingly for such a common verb, happen is a comparatively recent addition to the English language. Old English had a number of verbs denoting ‘occurrence’, all long since defunct, including gelimpan and gescēon, and in the 13th century befall began to be used for ‘happen’, but the first signs we see of the coming of happen are when English acquired the noun hap ‘chance, luck’ in the 13th century.

It was borrowed from Old Norse happ, a word of uncertain ancestry but probably related to Old Slavic kobu ‘fate’ and Old Irish cob ‘victory’, and represented in Old English by gehæp ‘fit’. In the 14th century it began to be used as a verb meaning ‘happen by chance’, and hence simply ‘happen’, and before the century was very old it had been extended with the verbal suffix -en to happen.
=> happy, perhaps[happen etymology, happen origin, 英语词源]
happen (v.)youdaoicibaDictYouDict
late 14c., happenen, "to come to pass, occur, come about, be the case," literally "occur by hap, have the (good or bad) fortune (to do, be, etc.);" extension (with verb-formative -n) of the more common hap (v.). Old English used gelimpan, gesceon, and Middle English also had befall. In Middle English fel it hap meant "it happened." Related: Happened; happening. Phrase happens to be as an assertive way to say "is" is from 1707.