In the literal sense, a bag, that one would use to jerk off into, when in an environment that would not appreciate being cumstained - see cumstain, for example, a friends car. An old sock could be used as a "flogbag".

The word is more commonly used however to as a label to describe someone you don’t like, someone who has done something stupid or behaves in a generally annoying and foolish manner.

(noun) A successful type of theft which results in an acceptable, impressive and rewarding payday for the protagonist.