Apart from being a woodcutting tool and a favoured edged weapon of both Vikings and barbarians all around the Medieval world, "axe" is also a slang term for an electric guitar or bass, usually used within the rock and heavy metal communities, who often like to view themselves as the modern world's very own barbarians -- and, in the case of certain Scandinavian black metal acts, its very own Vikings.
Gene Simmons, bass player of the Good Old Rock Band KISS, often played a bass shaped like a large double-headed battle axe with strings attached. The slang term 'axe' predates Simmons' axe bass, so it is likely that the slang term was what inspired him to have this instrument built.
Not all guitars qualify as axes, but whether or not a guitar gets to bear the axe moniker is up to the
axeman guitarist who wields plays it. Usually, your uber-cool new B.C. Rich model is an axe, whereas your old, trashed Strat copy or your pet acoustic is a shovel, perhaps even a shit shovel. Generally, your axe gradually turns into a shovel as your liking for the instrument wanes.