A MacJagger is someone who randomly makes references to television shows, movies, songs, and other media throughout the day.
Often when making said references, the MacJagger's audiences won't even realize what he/she is referencing to and will wind up confused.
Shane: Hey, I had some cake after lunch today, it was tasty.
Connor: The cake is a lie.
Patrick: Don't mind Connor, he's just a MacJagger...