A person who smokes weed, listens to reggae (usually just the music of Bob Marley), and claim to be a rasta though they haven't the slightest clue about the actual religion. These people tend to be caucasion, use the term I and I (the speaker and G-d), and use the word Jah (a name for G-d) a lot. Also, impostafarians tend to listen to crappy bands that mix reggae, rap, and rock poorly. Such as Sublime.
Impostafarian: Thank Jah for the herb and Sublime.
Normal Person: Sublime sucks.