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.