This sign is commonly understood to mean "pea
Note: it is VERY similar to the sign for GOVERNMENT. But there are some small differences. There sign for pea brain makes contact, comes off the head, twists, and then makes contact again. The sign for "government" draws a circle in the air and then makes contact with the forehead.
(ASL researchers suspect there may be possible connections between the sign for "pea brain" and the sign for "governor." I'll leave you to draw your own conclusions.
(side view) MORON
Also see: INCOMPETENT
American Sign Language University ™ ASL resources by Lifeprint.com © Dr. William Vicars