A small, very low power air pistol. A .177 air pistol with a reciprocating piston barrel (you push the pistol against the ground to cock it)
Now on sale again, aparently due to popular demand.
Traditionally sold to small boys along with some feathered darts, pellets and corks.
--CGS 09:36, 16 May 2005 (BST)
Although to be fair, most just use it to describe any issue rifle. G3, FAL as examples - generally not accused of being of low power.
Also as I once wrote before, well worth knowing for the Torygraph crossword.