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default on (something)
To fail to repay a loan on something (which often results in the loss of that thing). If you default on your car loan, your car might get repossessed.
default on something
to fail to act in some way regarding something, such as failing to make a payment, thereby losing one's right to the thing in question. You are not going to default on your loan, are you? She defaulted on her mortgage payments and lost the house.