In Heb. the word sometimes tr. “fellow” include the basic rēa. The feminine of this word has been tr. “fellow” in Judges 11:37. In the following v. it is tr. “companion.” Also the form hábēr alternates between the tr. “companion” and “fellow” as in Psalm 45:7; Isaiah 44:11 and Ezekiel 37:19. “Knit together as one man” is one expression comprehending the deeper significance of the word. Another Heb. form amith (Zech 13:7) may have a more abstract meaning as “man of my fellowship.”
The Gr. forms include plēsion used in the feminine, but tr. “fellow”; and métochos (Heb 1:9), a quotation from Psalm 45:7, used in the sense of companion.
Another basic use of the word relates “fellow” to a person or persons, and often is used as an expression of contempt. The Heb. ĭsh (1 Sam 29:4) is at times tr. “fellow.” The demonstrative zĕh (this) is also used to express contempt.
International Standard Bible Encyclopedia (1915)
Meant originally a "partner," from fe, "property," and lag, "to lay," then "a companion," "an equal," "a person or individual," "a worthless person."
Fellow-citizen, Fellow-disciple, Fellow-heirs, Yokefellow, etc. In composition, "fellow" always means partner or companion.