Rashi on Deuteronomy 29:18/רש"י על דברים כ״ט:י״ח

1והתברך בלבבו AND HE BLESS HIMSELF IN HIS HEART — The word והתברך has the meaning of “blessing”. In his heart he will imagine for himself a blessing of peace, saying, “These curses will not come upon me, I shall have peace”.

2והתברך is benoir soi in old French, i.e. the verb is reflexive like והתגלח, “he shall shave himself”, והתפלל, he shall pray lit., he shall make himself the object of intercession.

3בשררות לבי אלך means IN THE VIEWS OF MY HEART WILL I WALK, שררות being of a root which means to look, to see, as (Numbers 25:17): “I shall see him (אשרנו), but not now”; it is as much as to say, I will follow what my heart sees good to do.

4למען ספות הרוה TO ADD DRUNKENNESS [TO THE THIRST] — In order that I may add punishment for him even for the sins he has committed until now inadvertently (for which the figurative expression in this sentence is הרוה, drunkenness: cf. the following passage in Rashi), and which I used to overlook; — but now he causes Me to combine them with those committed with premeditation and to exact punishment from him for everything. Onkelos, too, rendered it in a similar sense: “in order to add for him the sins of inadvertence to those of premeditation”, which can only mean. “In order that “I” may add sins of inadvertence to those of presumption”.

5הרוה DRUNKENNESS figuratively describes the condition of a שוגג, one who acts inadvertently. The expression is an apt one because he acts like a drunken man who does things unwittingly,

6הצמאה THE THIRST aptly describes the attitude of one who acts wittingly and out of desire.