Commentaire sur La Genèse 25:28

בפיו [THERE WAS HUNTING] IN HIS MOUTH — Understand this as the Targum renders it: in Isaac’s mouth (i. e. Isaac ate the venison he brought home). But its Midrashic explanation is: there was hunting in Esau’s mouth, meaning that he used to entrap and deceive him by his words (Genesis Rabbah 63:10).

BECAUSE THERE WAS VENISON IN HIS MOUTH. The commentators³⁰Ibn Ezra and R’dak. explained it as meaning either that he [Esau] gave venison into Isaac’s mouth, or that he brought Isaac venison. The act of giving or bringing is thus missing from the verse. Similarly: The set time which Samuel;³¹I Samuel 13:5. Here the word “appointed” is missing, the meaning of the verse being “at the set time which Samuel had appointed.” But the Lord — presented or made — me as a mighty warrior.³²Jeremiah 20:11.
It is possible to explain that Isaac loved Esau because there was always venison in the mouth of Isaac. All day he would desire to eat the venison, and it was always in his mouth. He would not eat anything else, and Esau was the one who brought it to him, as Scripture said, A cunning hunter.³³Above, Verse 27.
In my opinion the correct interpretation is that it is a metaphor which tells us that Esau, in the mouth of his father, was a hunter,³⁴Instead of calling him by his name, Isaac would call him “hunter.” as a person is surnamed by his constant occupation. Similarly, Thy habitation is in the midst of deceit;³⁵Jeremiah 9:5. Meaning “in the midst of people of deceit.” But the verse refers to the people by their constant practice. likewise, But I am all prayer.³⁶Psalms 109:40. Meaning, “But I am a man of constant prayer.” And so they said in Bereshith Rabbah,³⁷Bereshith Rabbah 63:15. “Good meat for his mouth, good drink for his mouth.”³⁸If he would find good meat or drink, he would bring it to his father. This Midrash thus indicates that Esau was constantly engaged in bringing food and drink to his father. Hence Isaac came to call him “hunter” because of his steady preoccupation with bringing him food.

כי ציד בפיו, the plain meaning corresponds to Onkelos, i.e. because he brought him venison to eat.