Back in the 70s, musicians like Edgar Winter came up with the idea of slinging a synthesizer around their neck on a strap so they could walk around like a guitar player.
This inspired some keyboard manufacturers to come up with what was called a keytar.
To visualize things another way, the keytar concept can be described this way:
The keytar has never been much more than a curiosity, although some musicians (included Lady Gaga) still like to use one. There’s enough interest in the thing for Roland to introduce the AX-Edge, which looks like it could also double as an executioner’s axe.
Read more here.