coincidental (adj.)

1560s, "exactly corresponding, having the same nature or character;" 1590s, "happening at the same time, concurrent," from French coincident, from coincider,from Medieval Latin coincidere, literally "to fall upon together," from assimilated form of  Latin com "with, together" (see com-) + incidere "to fall upon" (from in- "upon" + combining form of cadere "to fall," from PIE root *kad- "to fall").

suffix forming adjectives from nouns or other adjectives, "of, like, related to, pertaining to," Middle English -al, -el, from French or directly from Latin -alis (see -al (2)).

"in a coincidental manner, concurrently," 1823, from coincidental + -ly (2). The earlier adjective was coincidently (1620s).