When someone has been idiotic, foolish, or incredibLy fucking stupid.
"Hey Jim! Wheres my hat?"
"On your head you spanner monkey!!!"
A largely useless individual whose only career prospects are to work in maintainence departments painting walls and replacing light-bulbs. May have questionable habits such as an excess love of porn, language that would make a soldier blush and a equally useless son.
"Why can't one of those spanner monkeys come up here to fix the lights?"