Do you call someone who is a bit Mad (not in an institutional way) a "loose cannon"?
Around here, we call them "Fred"
.
But really, a loose cannon would be somone whose reactions won't neccessarily be those required by the "higher ups" (Bosses and so on). They would tend to do something different to what their company (or Platoon Leader, etc) might want.
The expression comes from the days of Naval Warfare. Imagine: You have loaded your cannon to fire at the French/French/Spanish/French/Dutch/French/French etc. It takes a few seconds for the fuse to burn down to the gunpowder. Your cannon is not properly secured and the ship rolls on the waves (of course). A loose cannon would roll backwards and forwards and when it fires, that cannonball could go anywhere.
Hence the term "loose cannon".
Some other expressions used in the UK for "mad" are:
Loopy
Crazy
A bit of a loon
More expressions:
Dead Chuffed
Top notch
U/S
Bodge Job
Any of those familiar in the USA?
In the USA would you call the doctor who analyses a mad person a "Trick Cyclist"?