Ack! is a meme spammed across 4chan boards, participially /tv/ and /pol/. The earliest known use of the meme was on September 8, 2021 by an anonymous user (who, in this case, was the OP of the thread) on /tv/. The thread depicted a GIF of a trans woman, who barely looked like a woman, hanging herself in hellfire. The description simply read “ACK!”. Variations of the meme include /pol/ack and yellow dog (who is meant to symbolize a mod).
A meme originating from 4chan and soyjak.party around late 2021 used to deride another user by indicating that they're suicidal, e.g. that they were hung in the middle of typing their sentence. It immediately ends the quoted sentence, is often put in place of vowels near the end of a sentence for maximum onomatopoeification, and in addition it's almost always in greent-ACK!
Anonymous No.1: rope tranny you will never be a woman
Anonymous No.2: >rope tranny you will never be a wom-ACK!