A "harry potter
" is not a person, but in fact an action. If you go through any of the harry potter movies or books, you will notice that the main character - harry potter cannot go 5 minutes without doing something foolish, stupid or downright embarassing.
Therefore a "harry potter" is an action that is foolish, stupid, ignorant, embarassing, funny or reflective of something harry potter would do.
E.g. of in the movie, harry potter and the goblet of fire
and her friends on the ravenclaw
table giggle and look towards harry. Harry notices this and smiles back, only to forget to his horror that he has milk in his mouth, it spills everywhere. Laughter commences.*