From sun and the base of basis; to walk (figuratively, transpire) together, i.e. Concur (take place) -- be(-fall), happen (unto).
see GREEK sun
see GREEK basis
1. to walk with the feet near together.
2. to come together, meet with one; hence,
3. of things which fall out at the same time, to happen, turn out, come to pass (so occasionally in the Sept for קָרָה and קָרָא); as very often in Greek writings (the Sept. Genesis 42:4; Genesis 44:29), συμβαινει τί τίνι, something befalls, happens to, one: Mark 10:32; Acts 20:19; 1 Corinthians 10:11; (1 Peter 4:12); 2 Peter 2:22; τό συμβεβηκός τίνι, Acts 3:10 (Susanna 26); absolutely, τά συμβεβηκότα, the things that had happened, Luke 24:14 (1 Macc. 4:26; (Josephus, contra Apion 1, 22, 17)); συνέβη followed by an accusative with an infinitive it happened (A. V. so it was) that, etc.: Acts 21:35 (cf. Winer's Grammar, 323 (303)), examples from secular authors are given by Grimm on 2 Macc. 3:2.