'Jump seat' definitions:

Definition of 'jump seat'

noun

A folding seat in an automobile

From: GCIDE

Jump \Jump\, n.
1. The act of jumping; a leap; a spring; a bound. "To advance by jumps." --Locke. [1913 Webster]
2. An effort; an attempt; a venture. [Obs.] [1913 Webster]
Our fortune lies Upon thisjump. --Shak. [1913 Webster]
3. The space traversed by a leap. [1913 Webster]
4. (Mining) A dislocation in a stratum; a fault. [1913 Webster]
5. (Arch.) An abrupt interruption of level in a piece of brickwork or masonry. [1913 Webster]
6. A jump-start; as, to get a jump from a passing mmotorist. [PJC]
From the jump, from the start or beginning. [Colloq.]
Jump joint. (a) A butt joint. (b) A flush joint, as of plank in carvel-built vessels.
Jump seat. (a) A movable carriage seat. (b) A carriage constructed with a seat which may be shifted so as to make room for second or extra seat. Also used adjectively; as, a jump-seat wagon. [1913 Webster]