on the off chance
if you do something on the off chance, you do it hoping for a particular result, although you know it is not likely

[TahlilGaran] Dictionary of Contemporary English ▲

on the off chance
because it is possible but unlikely.
I bought a first edition of the book on the off chance that it might be valuable someday.

[TahlilGaran] English Idioms Dictionary ▲

on the off-chance
if you do something on the off-chance, you do it because you hope you will get or find something or someone, even if it is not very likely.
I don't think he works in the shop on Saturdays, but you could stop by on the off-chance.

Journalists often investigate film stars' private lives on the off-chance that they might find something scandalous.
(often + that)
She flew in from New York on the off-chance of getting tickets to see Becker play his last match at Wimbledon.
(often + of + doing something)

[TahlilGaran] English Idioms Dictionary ▲

on the off-chance
in case something may happen, the slight possibility that something may happen
I asked the salesman if he had a part for my computer on the off-chance that he may have one in his store.

[TahlilGaran] English Idioms Dictionary ▲