To rest with the torso vertical or upright and the body supported on the buttocks which rest on a chair, the ground, etc.
Not to be confused with the movement of the sign for SIT or SIT-DOWN (one movement for this verb) and CHAIR (twice movement for this noun).
If you happen to see an ASL speaker signing more than one movement in a particular context, it's a plural verb (for example, telling everyone to sit down). That is, SIT-DOWN++++.
Review 101 words. Refresher.