The banjolele (a.k.a the banjulele banjo, or the banjo uke) was invented in 1917 by Alvin D. Keech. It features ukulele strings on a miniature-sized banjo frame. The instrument became very popular with vaudeville performers, because it was small like a ukulele, but had more volume. The instrument reached the peak of its popularity in the 1920s and 1930s, and was popular enough, obviously, that it showed up on sheet music (though I never saw music with banjo-uke tabs, just chord symbols). And it turns out that the instrument has not gone away. There are lots of people out there still strumming away on their banjoleles.
Here's a demonstration of someone playing a banjolele:
Probably the most famous champion of the banjolele was Beatle George Harrison. You can see a video of him playing one here. Harrison used to give away ukulele and banjoleles, and often had two on his person at all times—so he could play with someone else. At Paul McCartney's concerts, Paul often plays George's songs on the ukulele as a tribute.
Vocab: strings, banjo, cove