Scottish phrase used to describe weather conditions such as Heavy rain showers for short periods of time. Rain that appears out of nowhere and soaks you through to the bone quickly and then disappears without a trace usually followed by more of the same throughout the day.
Aye I got caught by a ninja shower. Oh ya hoor sir there's ninja showers happening. Got soaked by a ninja shower.
If you're one of those whiny adolescent morons who threatens complete strangers with violence over Xbox Live despite the fact you haven't even got hairs on your chin, don't play Ninja Gaiden Black. The sudden jolt back to reality may annihilate you.