Deuteronomy 32:15

עבית has the meaning of “thickness” (stoutness), it therefore means: “ Thou hast waxed fat”. כשית is the same as כסית (‎ס and ש interchanging). It has the same meaning as in Job 15:27 “Because he hath covered (כסה) his face with his fatness”— like a person who is full of fat inside and whose flanks therefore lie in folds outside, as it goes on to state, (Job 15:27) “and be made folds of fat on his flanks” (Sifrei Devarim 318:6). כשית — There exists a Kal form (as is this word) in the sense of covering, as (Proverbs 12:16) “And a prudent man is covered with (וְכֹסָה) shame”. If it had written כִּשִּׂיתָ with a dagesh in the ש. it would imply, “Thou hast covered others (or other things)” (i.e., it would be transitive), as in the text already quoted, “For he hath covered (כִסָּה) his face”. וינבל צור ישעתו AND HE LIGHTLY ESTEEMED THE ROCK OF HIS SALVATION — This means, He showed contempt for Him and scorned Him, as it is said, (Ezekiel 8:16) “[and, behold, … there were … men] with their buttocks towards the Temple of the Lord” — surely, there can be no greater contempt than this (see Rashi on this verse) (Sifrei Devarim 318:9).

But Yeshurun grew fat, and kicked: thou art grown fat, thou art become thick, thou art covered with fatness; then he forsook God who made him, and lightly esteemed the Rock of his salvation.

וַיִּשְׁמַ֤ן יְשֻׁרוּן֙ וַיִּבְעָ֔ט שָׁמַ֖נְתָּ עָבִ֣יתָ כָּשִׂ֑יתָ        וַיִּטֹּשׁ֙ אֱל֣וֹהַּ עָשָׂ֔הוּ וַיְנַבֵּ֖ל צ֥וּר יְשֻׁעָתֽוֹ׃