[fall back on] or [fall back upon] v.
1. To retreat to.
* /The enemy made a strong attack, and the soldiers fell back on the fort./
2. To go for help to; turn to in time of need.
* /When the big bills for Mother's hospital care came, Joe was glad he had money in the bank to fall back on./* /If Mr. Jones can't find a job as a teacher, he can fall back on his skill as a printer./