cer! - 'go'! It does not really mean 'get!' on its own.
Possibly Duo was thinking of cer o 'ma! - 'get out of here! or cer o 'na! - 'get out of there!'.
In Welsh, the prepostions that can come after a verb or verb-noun are essential to the meaning of the two words together - you cannot always split them and retain the same meaning:
mynd - to go
mynd i - to go to (somewhere, to do something)
mynd â - to take
mynd o - to get out of, to leave