SILLY: The American Sign Language (ASL) sign for "silly"
 

The sign for "silly" wiggles a "Y" hand in front of your face. The movement is a double twist. Sort of like the "hang loose" gesture or the sign for YELLOW (but done in front of the face).
 

SILLY:

Memory Aid: "Homer Simpson has a silly yellow face."

You can modify the sign "SILLY" to mean, "that's ridiculous" or "he is a fool." The modified version uses a single large sweeping motion with a bit of a twist in the wrist.

SILLY / foolish / ridiculous:

Another variation of SILLY is FRIVOLOUS or PLAYING AROUND:

Sample sentence: YOU THINK HE/SHE/THEY-[singular] TEACHER SILLY? (Do you think the teacher is silly?)
 

Notes: 

Also see: DUMB

Also see: STUPID