holding horizontally in one's mouth; holding at the side of one's mouth
Describes holding something (often a pipe, cigarette, or small object) sideways in the mouth, or at the corner of the mouth. Used adverbially.
He held his pipe horizontally in his mouth and slowly exhaled smoke.
The cat ran off holding a small fish sideways in its mouth.
Compound of 横 (yoko, 'side, horizontal') and 銜え (kuwae, 'holding in the mouth'). The exact historical derivation is uncertain, but the word is transparently formed from these elements.