Acknowledge, i.e. make it appear that he owns him. Double portion; for the phrase, see 2 Kings 2:9 Malachi 13:8; and for the thing, see Genesis 25:31 1 Chronicles 5:1. The beginning of his strength, i.e. the first evidence of his manly strength and ability for procreation.
Related Commentaries of Deuteronomy 21:17
Deuteronomy 21:17
17 But he shall acknowledge the son of the hated for the firstborn, by giving him a double portion of all that he hath: for he is the beginning of his strength; the right of the firstborn is his.