A.T. Robertson Commentary Hebrews 7:10

"for he was yet in the loins of his father, when Melchizedek met him." — Hebrews 7:10 (ASV)

In the loins of his father (εν τη οσφυ του πατρος). Levi was not yet born. The reference is to Abraham, the forefather (πατρος) of Levi. This is a rabbinical imaginative refinement appealing to Jews.