side-mouth
verbUK /ˈsaɪd maʊθ/
US /ˈsaɪd maʊθ/
To talk out of the side of your mouth so others can't hear.
He side-mouthed a secret to his friend during class.
OTHER MEANINGS
2. To speak in a quiet, secretive way to someone nearby.
She side-mouthed the answer to me during the test.
3. To say something without moving your lips much.
The spy side-mouthed the code word to his partner.
4. To speak in a low, unclear voice.
He side-mouthed his complaint so the teacher wouldn't hear.