Happen

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late Middle English (superseding the verb hap ): from the noun hap1 + -en1.

wiktionary

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From Middle English happenen, hapnen, augmented from Middle English happen(“to come to pass, happen”), perhaps from Old English hæppan(“to move accidentally, slip”) and/or from Old Norse *happa, *heppa, from Proto-Germanic *hampijaną(“to fit in, be fitting”). Equivalent to hap(“a chance, occurrence, byfall”) +‎ -en(verbal suffix).

etymonline

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happen (v.)

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.