(Not a teacher)
Urban Dictionary can be useful at times. If you don't know what it is, it's just a website where people create definitions for words and phrases commonly found in language that is slang/vulgar/colloquial/non-standard etc. While I wouldn't suggest you use it frequently, it can be helpful when the word/phrase is something a 'normal' dictionary wouldn't have. So, from Urban Dictionary:
When everyone jumps on top of one unfortunate soul, they 'pile on'.
The much less wimpy sounding version of bundle.
'Look, Chris is asleep. Pile on!'
It is also used as a noun - 'a pile on'.