YOKEFELLOW (Gr. syzygos, yoked together). A common word among Greek writers referring to those united by close bonds, as in marriage, labor, etc. It is found only once in the NT (Phil.4.3) and the meaning here is not clear. Some feel that Paul refers here to a fellow worker; others think the word is a proper noun, Syzygos.

YOKEFELLOW (σύνζυγος, yoked together). A word found only in Philippians 4:3: “And I ask you also, true yokefellow, help these women.” The Gr. word σύνζυγος is taken by some interpreters as a proper name, although it is not found anywhere else as such; and many suggestions have been made in regard to the identity of the person: Luke, Lydia, Epaphroditus, Barnabas, Silas, Timothy, Paul’s wife. It may be a way of describing the leader of the church at Philippi. Conjectures are legion.

International Standard Bible Encyclopedia (1915)

The word is used by Greek writers of those united by any bond, such as marriage, relationship, office, labor, study or business; hence, a yoke-fellow, consort, comrade, colleague or partner.

(1) In the New Testament it occurs once only (Php 4:3): "I beseech thee also, true yoke-fellow." Most interpreters hold that Paul here addresses some particular but unnamed person, who had formerly been associated with him in the work of the gospel in Philippi. Many guesses have been made in regard to the identity of the unnamed "yoke-fellow," and these names have been suggested: Luke, Lydia, Epaphroditus, each of whom had in one way or another some connection with Philippi.

(2) Renan has suggested that yoke-fellow means Lydia (Ac 16:14,15,40), and that she had been married to Paul. But the fact that the adjective gnesios, "true," qualifying "yoke-fellow" is masculine and not feminine shows that it is not a woman but a man who is referred to. Renan’s suggestion is an unworthy one, and is quite devoid of proof. It is a mere fanciful and unsupported creation of the Frenchman’s brain. Renan’s idea is a modification of an opinion which is as old as Clement of Alexandria, that Paul here referred to his own wife. But this conjecture is contradicted by the statement of the apostle himself, that he had not a wife (1Co 7:8; 9:5).

(3) There is still another way of interpreting "yoke-fellow," and probably it is the right one. Some expositors take the word as a proper name. Among these Westcott and Hort print "Sunzuge," in the margin. In favor of this interpretation there is much to be said, especially the fact that the word is found in the very midst of the names of other persons. The names of Euodia and Syntyche are mentioned immediately before, and that of Clement follows immediately after the true yoke-fellow. The meaning therefore is probably, "I beseech thee also, true Synzygos," i.e. I beseech thee, who art a genuine Synzygos, a colleague rightly so called, a colleague in fact as well as in name. It is obvious to compare the way in which the apostle plays upon the name Onesimus, in Phm 1:11.