Best definition
An example used to confuse a Grammer Nazi

Person: The trap is set, but who’s tricking who?

Grammer-Nazi: It’s actually whom. It’s who’s tricking whom.

Person: Whomst’d’ve’s’nd do you think you are?

Grammer-Nazi: …
whomst’d’ve’s’nd: define #2
A popular meme posted by obscure Instagram accounts, gaining popularity in early 2017. People use it to sound smart.
Jimmy: Tom, do you know who did this?

Tom: You mean, do I know whomst’d’ve’s’nd did this? Yes, yes I do.

Jimmy: Stfu.