Cognate to the word ἀπόστολος (apostle). The I is emphatic: “It is I that send you forth.”
Wise (φρόνιμοι)
So A.V. and Rev. Denoting prudence with regard to their own safety. Wyc., wary.
Harmless (ἀκέραιοι)
Lit., unmixed, unadulterated. Used of wine without water, and of metal without alloy. Hence guileless. So Luther, without falsity. Compare Rom 16:19; Phi 2:15. They were to imitate the serpent's wariness, but not his wiliness. “The presence of the wolves demands that ye be wary; the fact that ye are my apostles (compare “I send you”) demands that ye be guileless” (Dr. Morison on Matthew).