Sleeve gun

Sleeve gun is a generic term for small firearm designed to be concealed under a long-sleeved coat or jacket – in fictional examples there is often a mechanism to extend it out into the hand to fire.
Real examples.
The "sleeve gun" was developed during World War II by Station IX of the Special Operations Executive. It was essentially a version of the noise-suppressed Welrod pistol, minus the pistol grip – and produced in both .32 ACP and 9×19mm. 
In films and television.
Though designs vary, most fictional sleeve guns involve a small conventional pistol on a sliding or telescoping rail, which quickly releases the weapon into the hand for firing, either by a trigger mechanism, or just the sudden movement of the forearm. 
Such weapons have appeared in films shows TV: