play possum
play possum {v. phr.}, {informal}
1. To pretend to be asleep.
Johnny seemed to be fully asleep, but his mother knew that he was playing possum.
2. To stay quiet for self-protection; try to escape attention by inactivity.
The rabbit played possum under the bushes, hoping the hunter would not see him.
Bob played possum when the teacher looked around.
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Source: A Dictionary of American Idioms