Pull the rug out (from (under) one) phrase
To abruptly stop helping or providing support or assistance to someone, especially when that person is in a troublesome situation.
John pulled the rug out from under Mary. I saw him catch her hand, but then he dropped.
Mary promised to finance my project, but when I went halfway to completing it, she pulled the rug out from under me.
The verb "pull" must be conjugated according to its tense.
The image is undeniably clear, but a more common practice, it would seem, would be the schoolboy trick of pulling a chair away from someone who is about to sit down. It is "rug", however, that became part of a common turn of phrase, originating in the mid-twentieth century. Time used it in an article about labor and the economy in 1946
To be dishonest or deceptive and no one can believe
No one believes what she said. She couldn't lie straight in bed.