@sssr
Pretty sure whaling on something just means to beat it repeatedly, so whaling on a sandbag would be to just punch a sandbag repeatedly like boxers do (but without looking like you know what you're doing)
Jesus you guys are taking this too seriously, ok the friend had like one really creepy moment, but then you're acting as if every moment after this is something that he schemed for his own benefit, smh he was just too lazy to get his umbrella out