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1.
One who has the ability to think for themselves and uses it.
If you had to read the example, you are not a thunker!

by Robert L Wooten September 27, 2006
 
2.
To make a large mistake like to shoot a 8 on a par 4 golf hole, or to totally screw something up, like ruin a term paper for school
Man, thats the worst thunker i have ever shot in one round of golf
by Johannathon June 17, 2005