A fit of pique, a huff. To be "in a strop" is to be discontent, disgruntled or just plain angry. Normally refers to a silent state of ire, which only expresses itself when the stroppy one is shamelessly provoked by unreasonable questions such as "Would you like a cup of tea?"
As indicated in the above construction, the adjective "stroppy" describes somebody who is in a strop, or who demonstrates behaviour associated with such a state.