American Sign Language:  "fake"

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The sign for "fake" is normally listed under the concept of FALSE.
To do the sign for "false," or "fake" -- move your "index finger" past your nose. Your index finger doesn't touch the nose, it just comes close. The movement can be in the wrist or it can be more in the elbow as I show here.  The movement is NOT in the knuckle.
 

FAKE (or false)

 

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See: FALSE

If you mean "fake" as in "He is a fake!" (as in "a hypocritical person") you should use the sign for "HYPOCRITE."