[Aussie Mike]

Its surprising how many of these phrases have military origins. I didn't know the Petard one but I'll file that away.

How about "caught napping" you would think it meant getting caught having a sleep when it actually refers to napping flint. A musket man finding his flint lock not making enough sparks to ignite the powder would chip away at the edge of the flint with a tool to reveal fresh flint. Not a good thing to be doing when the enemy is sudenly upon you.

This also leads to the phrase "flash in the pan" which actually refers to the powder in the flint locks pan ignighting but not setting off the charge behind the musket ball.

Cheers