Sorry, I thought it was funny.
There was an episode where a ghost hunter is murdered during a night time ghost investigation, and Castle keeps bringing up the possibility that he may have been killed by a ghost, which the detective soundly rejects. Castle mentions the ghost a dozen times...to the annoyance of the detective. When he starts to say something about it again and just about says the word "ghost" once more, the detective stops him before he says the word "ghost" and warns him: "If you say 'ghost' one more time..."
So Castle finishes his sentence saying "Aparation American"
....you know, like African American.
Oh well...I just thought it was a funny term.