high sign
high sign {n. phr.}, {informal}
A silent signal of recognition, greeting, or warning; an open or secret signal between two persons. — Used with "get" or "give".
The Joneses saw us across the hotel dining room and gave us the high sign.
John could see that Grace wanted to tell him something, but he got her attention and frowned. She got the high sign and waited until the teacher had moved on before speaking.
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Source: A Dictionary of American Idioms
