C. F. Keil and Franz Delitzsch on Ruth 3:9

Rth 3:9

In answer to his inquiry, “Who art thou?” she said, “I am Ruth, thine handmaid; spread thy wing over thine handmaid, for thou art a redeemer.” כּנפך is a dual according to the Masoretic pointing, as we cannot look upon it as a pausal form on account of the position of the word, but it is most probably to be regarded as a singular; and the figurative expression is not taken from birds, which spread their wings over their young, i.e., to protect them, but refers, according to Deu 23:1; Deu 27:20, and Eze 16:8, to the wing, i.e., the corner of the counterpane, referring to the fact that a man spreads this over his wife as well as himself. Thus Ruth entreated Boaz to marry her because he was a redeemer. On this reason for the request, see the remarks in the introduction to the chapter.