When you have something incredibly important to say to someone and you're nervous, but when you tell them you realize they are incredibly clueless.
Taken from Lost, season 5, episode 4, when Jack confronts Carole about wanting back her grandson and she has no clue what he's talking about.
Jack: I understand that you feel the need to do this, but I need you to know, that everything that Kate and I have done, it was for Aaron.
Carole: ...who's ... aaron?
Guy: So I came out to my parents last night, and they were like "...who's aaron?" You know what I mean?
Girl: Oh Ha ha! No way bra they're so cluless.