mixing rice and side dishes in one's mouth
Colloquial term describing the act of putting rice and a side dish into the mouth together and mixing them while eating, rather than eating them separately. Often used humorously or critically in discussions about table manners.
Mixing rice and side dishes in your mouth is sometimes said to be bad manners.
彼はいつも口内丼をしている。
He always mixes his rice and side dishes in his mouth.