Single Jack Hammer: A single jack (3- to 4-pound head with short
handle) hammer can be used with a star drill to punch holes in rock.
The single jack can also be used to drive bridge spikes, and other uses
that are too demanding for a regular claw-hammer, but do not require
the heavy duty blows of a sledge.

I beleive a single jack was called that because it was used with a
single hand, a double jack required two hands...........why they're
called "jacks" I don't know

cheers
Bob