Systematic punching many times, or hitting with a blunt instrument, to enforce an order, inflict punishment, prevent the victim from resisting for a while, or release anger. Often two or more men take part, sometimes ith one or two of them restraining the victim.
In some out-of-date dictionaries, "to alarm by a sudden attack"
Beating oneself up about something: feeling badly or guilty about it
Beat-up (adjective), of a car: battered by time and usage
In WW2 air pilots' usage, repeatedly bombing a military target or targets