It's not the bolt, it's the fitting the bolt secures. It resembles a
banjo-to some one. I've seen worse analogies.

Ever look at the fitting it bolts down? Maybe you get the impression it
looks kinda like a little banjo? - Y'might have to squint just right to
catch the resemblance, but for me, and many others, it's there.

Presto... That'd be why it's called a banjo fitting, and the hollow and
pierced bolt designed to hold it in place is called a banjo bolt.

because it goes thru a fitting that is round and has a hole thru it. it's
used to connect a hydraulic
(usually a hose to something.)
sammmmm