Hehe. It happens to the best of us and I mean everyone even if people say they don’t. Just ignore it and when he says it distract your child with something else. If you pay attention to it they will do it more.
It's best to ignore I've learnt. When I've drawn attention to it he finds it funny and keeps saying it. I've found if i don't give any response at all when he comes out with words like this he's heard accidentally then he gets bored and he just forgets about them.
Just ignore it or try change it into another word. Say “Oh that’s not what we say, we say Whoopsie Daisy! That’s way more fun!” Or “Oh shucks!” (If they slip it out in public we like to just repeat after them “Oh shucks” a few times so that people know or think that’s what they’ve said. My daughter keeps saying what sounds like “fucking” whilst she’s playing pretend but I have no idea where she’s picked it up or if that’s what she’s actually saying so we’re ignoring or trying to find the word she actually means!