You are cutting yourself off at the footPosted: August 15, 2016
The speaker was telling someone that his statement was hurting his own argument. It is a nice blend of “shoot yourself in the foot” (foolishly harm one’s own cause) and “cut (someone) off at the knees” (thoroughly humiliate or squelch). Any frequent reader of this blog will know that idioms containing body parts are often confused and mixed. In this case, feet and knees are the culprits. Shooting and cutting, both action verbs that involve penetration, also seem to play a part in this malaphor. Many thanks to Lara Hayhurst Compton for saying this and Jody Compton for sending it on!
If you liked this one, and want more hands/feet/knees/ankles mix ups, get the book He Smokes Like a Fish and other Malaphors, available on Amazon at http://www.amazon.com/dp/0692652205.