(as) dumb as a fencepost

Very stupid. You took apart the entire washing machine just to unblock the pump? I swear, sometimes you can be as dumb as a fencepost, Larry. I used to think she was dumb as a fencepost when we were kids, but she grew up to be a particle physicist.
See also: dumb, fencepost

fencepost problem

A logic problem in which one must account for the boundaries around something in order to find the correct amount. It is typically explained using the imagery of a fence. A fence has X number of sections. It also has X + 1 or X - 1 (depending on how the fence is constructed) number of posts that act as boundaries for those sections. No, it's a fencepost problem, which means that you guys are one short in your calculation.
See also: fencepost, problem
Farlex Dictionary of Idioms. © 2015 Farlex, Inc, all rights reserved.