go back on one's word
To renege; break a promise.
Patrick went back on his word when he refused to marry Karen in spite of his earlier promise.
go back on
To turn against; not be faithful or loyal to.
Many of the man's friends went back on him when he was sent to prison.
The boy's father told him not to go back on his promise.
To fail to do necessary work; not work.
Grandfather's eyes are going back on him.