[fall back on] or [fall back upon] v. 1. To retreat to.
Theenemy 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 billsfor Mother’s hospital care came, Joe was glad he had money in the bankto fall back on.
If Mr. Jones can’t find a job as a teacher, hecan fall back on his skill as a printer.Значение идиомы fall back on