Lexical Definition

‎σύνειμι‎ sýneimi (soon'-i-mee) be with ^G:V
from ‎σύν‎ and ‎εἰμί‎ (including its various inflections)

be with.

• to be in company with, i.e. present at the time

Abbott-Smith

σύν-ειμι (< εἰμί to be), [in LXX: Je 3:20 (רֵעַ), al. ;]
to be with: Lk 9:18 (WH, mg., συνήντησαν), Ac 22:11.†

Abbott-Smith References

Liddell-Scott-Jones

σύνειμι,

(εἶμι ibo) go or come together, assemble, ἐς χῶρον ἕνα ξυνιόντες ἵκοντο [Refs 8th c.BC+]; συνιόντος ὄχλου πολλοῦ [NT]

__2 in hostile sense, meet in battle, [Refs 8th c.BC+]; of states, engage in war, [Refs 5th c.BC+]
__3 in peaceable sense, come together, meet to consult or deliberate,[Refs 4th c.BC+]; of a conspirator, σ. τοῖς φυγάσιν ἐπὶ καταλύσει τοῦ δήμου [Refs 4th c.BC+]; also of festive meetings, συνόδους συνιέναι [Refs 5th c.BC+]
__3.b of the council, σύλλογον ὃν εἶπες συνιέναι [Refs]
__4 σ. εἰς τὴν κοινωνίαν, of marriage, [Refs]; of copulation of animals, [Refs 4th c.BC+]
__II of things, gather, σ. ἀήρ [Refs 5th c.BC+]; of clouds, [Refs 4th c.BC+]; opposed to χωρίζεσθαι, [Refs]; σ. πρὸς αὑτήν recur, [Refs 5th c.BC+]
__II.2 of money, come together, come in, of revenue, [Refs 5th c.BC+]
__II.3 to be contracted, σ. καὶ ψύχεσθαι [Refs 4th c.BC+]
__II.4 of stars, come into conjunction, [Refs 4th c.AD+]; of the moon, συνιούσης, opposed to αὐξομένης, [Refs 6th c.AD+]

_{LSJ Scripture Refs: Luke.8.4}