This is a popular phrase used by those who speak ebonics (negroes). It is equivalent to "I am going to get" used by those speaking dialects other than ebonics.
Someone who uses an "imagine" insult more than once during an argument to strawman another person.
In a sentence:
You're a fucking imagitard. You can't even think of a creative or original thought you absolute bafoon.
Gamer 1: Imagine losing.
Gamer 2: I went top of the leaderboard and you got carried you brainlett.
Gamer 1: Imagine losing at the top of the leaderboard.
Gamer 2: No one likes you.
Gamer 1: Imagine being named justslappedyourmom69
justslappedyourmom69: You're an imagitard.