Getting in a strop

Why do we say "stroppy" - this is a mooring strop, why should it be associated with being nowty?
Anyway need one of these for the boat when she goes on the swinging mooring in the spring and the City Link man delivered some bits this afternoon and it was raining so a couple of eye splices are very therapeutic providing you get the first three tucks in correctly.
I like rope-work......