Cut the ground from under somebody’s feet
Definition 1 of 1
Pro Tip 1/3Unexpected Betrayal
The phrase often implies a sense of betrayal or disloyalty from someone thought to be an ally. 
Trusting them with our plans, only to have them compete against us, really cut the ground from under our feet.
Pro Tip 2/3Emotional Impact
It evokes feelings of shock, instability, and loss of support, much like a physical fall. 
After being denied the promotion, it felt like they'd cut the ground from under my feet.
Pro Tip 3/3Strategic Maneuver
Can indicate a clever or strategic move by others that puts someone at a disadvantage. 
They waited until I had made my argument to reveal the evidence, cutting the ground from under my feet.