A trekkie is a super nerd whose definition of a life is watching star trek all day and collecting star trek memorabilia. Lost their 'virginity' to a well-lubed sunglasses case lined with crab meat and plastic wrapping foil stuck between the bed and the mattress, with a facial photo of their favorite female character stuck to the pillow. Either that or their hand. First time they saw a chick's box was in a Penthouse magazine.
The documentary 'Trekkies' has countless examples of what makes a trekkie.