1A person linked to or associated with another, especially in some work or activity; an associate or partner in an undertaking; a fellow-worker. Also occasionally (literal): a draught animal coupled with another in a yoke.
2 historical, rare Specifically. A person joined in marriage to another; a husband or wife, a spouse.
3A thing which is closely connected to, associated with, or accompanies something else.
Early 16th century; earliest use found in The New Testament. From yoke + fellow.