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.
mid-15c., "chance, luck," verbal noun from happen (v.); meaning "an occurrence" is 1550s. Sense of "spontaneous event or display" is from 1959 in the argot of artists. Happenings "events" was noted by Fowler as a vogue word from c. 1905.
"possibly," late 15c., perhappes, with adverbial genitive, from earlier perhap (mid-14c.), from per, par "by, through" (see per) + plural of hap "chance" (see happen), on model of peradventure, perchance, etc. which now have been superseded by this word. Perhappons "possibly, by chance" is recorded from late 15c.
late 15c., "likely to happen," from Latin incidentem (nominative incidens), present participle of incidere "to happen, befall" (see incident (n.)). From 1620s as "occurring as a subordinate;" 1660s in literal sense "falling or striking upon."