look up any word, like the eiffel tower:
 
1.
An individual who believes in and spreads the various myths and hoaxes associated with the termination of the Mayan long-count calendar on and around December 21, 2012
John Doe: Hey sis, did you hear that Woody Harrelson's a 'twelver'?

Jane Doe: A what?

John Doe: A 'twelver'. You know, someone who believes the world is going to end on December 21, 2012.
by Hawkins' bitch July 31, 2011
 
2.
Duh; a twelve pack. usually of beer but can be soda.
Duude! He downed my whole twelver!
by jimmyvix May 14, 2010