Charles Ellicott Commentary Deuteronomy 32:18

"Of the Rock that begat thee thou art unmindful, And hast forgotten God that gave thee birth." — Deuteronomy 32:18 (ASV)

Of the Rock that begat thee. — “The Rock hath begotten thee forgetful, and thou hast forgotten God that travailed with thee” is another possible translation of this verse. The expression in the second clause is found also in Psalms 90:2 (a prayer of Moses), “Before the mountains were brought forth, while Thou wast yet in travail with earth and world, and from eternity unto eternity Thou art God!” The word which I have rendered “forgetful” is usually taken as a verb. But the verb is not found elsewhere (i.e., it is invented for this passage), and the word may possibly be an adjective.