'Cut one off, or loosen grip'. Meaning if someone asks you to do something for them and you accept, but you then just don't do it. We have an idiom that translates something around the lines of 'he cut me off or he loosened his grip'. They come from the idea of one holding on to a rope and the other person is below him and he cuts the rope or loosens his grip and he just falls. (It may be down to the fact that the person is putting more weight on the rope or because you can no longer hold on, so it's pretty much the same thing that he's going to burden you with something, and at that time you didn't have the impropriety and indecency to disappoint him/ it felt hard to say it to his face etc. so you just had to accept although falsely). That person has entrusted you and put confidence in you and then you just completely disregard him and forget about him for some reason or another. Similar idiom?
"He flaked on me." Someone who habitually fails to do as promised is a flake. It does not conjure the same mental image as "he loosened his grip", but it means the same thing idiomatically.
The most common usage of the verb "flake" is when someone agrees to meet someone else (especially on a date) and then does not show up. But the verb applies equally well to any kind of decision not to meet a commitment.
Another answer says "he let me down." While this idiom conjures a mental image similar to "he loosened his grip", its meaning is broader. "He let me down" really means "he disappointed me". It could mean "he refused to do as promised" or "he tried and failed" or "he did a bad job" or "he waited until it was too late" or "I was left unsatisfied."
But "he flaked on me" specifically means "he intentionally did not do what he agreed to do."