Motion diagram. This sign is outside the licensed video corpus, so the diagram is its full record here.
UNDERSTAND
“understand”
Hold a fist at the side of your forehead and flick the index finger up.
With a head shake: DON'T-UNDERSTAND. With raised brows it becomes the question 'do you understand?'
Where the sign comes from
UnknownNo reliable source. We'd rather say so than invent one.
At the forehead like the other mind signs. The popular explanation — a light bulb switching on — is a memory aid, not a documented origin; the sign predates that image in the language.
No sources — which is the point of the “unknown” tier. If you know of a documented account, the historical databases are where to check it.
The sign, taken apart
Every ASL sign is built from a handshape, a place it's made, and a movement. Learn to see these three and you can read and store any sign.
Starts as a fist with the thumb across the front; ends as index finger up, the rest closed.
Also: letter S
Made at the forehead.
One-handed.
Palm facing in.