Definition of dogpile:
(n) :
(Internet slang) The situation where many participants attack the same user (on a discussion forum or similar).
(v) :
(Internet slang) For many participants to attack the same user (on a discussion forum or similar).
(n) :
(US colloquial) A disorderly pile of people formed by jumping upon a victim.
(n) :
(figurative, US colloquial) Any similarly disorderly pile of people or things.
(v) :
(US colloquial, ambitransitive, often with 'on') To jump into a dogpile.
(v) :
(figurative, US, colloquial, ambitransitive) To pile on, to overwhelm in other senses.