This morning dd1 (2.3) wanted to go on the pc to watch mr tumble, i was busy (on mn) and i said 'no dd, wait', she looked up and said 'FFS'

Where did she hear that from? We try our best not to swear in front of our kids.
She must of heard it from dh because thats the sort of thing he say's.
How can i get it into her head that its naughty to say the F word?
I feel like a bad mummy 