fall back on
fall back on or fall back upon {v. phr.}
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.
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Source: A Dictionary of American Idioms