From etes (a clansman); a comrade -- fellow, friend.
Thayer's Greek Lexicon
Strong's 2083: ἑταῖρος
ἑταῖρος, ἑταίρου, ὁ (from Homer down), the Sept. רֵעַ ; a comrade, mate, partner (A. V. fellow): Matthew 11:16 (where T Tr WH τοῖςἑτέροις (which see 1 b., and cf. WH. Introductory § 404)); vocative in kindly address, friend (my good friend): Matthew 20:13; Matthew 22:12; Matthew 26:50.
Matthew 20:13 - N-VMS GRK:αὐτῶν εἶπεν Ἑταῖρε οὐκ ἀδικῶ NAS: to one of them, 'Friend, I am doing KJV: of them, and said, Friend, I do thee INT: of them he said Friend not I do wrong
Matthew 22:12 - N-VMS GRK:λέγει αὐτῷ Ἑταῖρε πῶς εἰσῆλθες NAS: and he said to him, 'Friend, how KJV: unto him, Friend, how INT: he says to him Friend how did you enter
Matthew 26:50 - N-VMS GRK:εἶπεν αὐτῷ Ἑταῖρε ἐφ' ὃ NAS: said to him, Friend, [do] what KJV: unto him, Friend, wherefore INT: said to him Friend for what [purpose]
