Two methods of surreptitious swearing:
1. Say the word “Arse’ole”, very loudly, then turn it into the beginning of the well-known song “O sole mio“. This is not strictly correct, as the words “O sole mio” aren’t sung to the actual “Just One Cornetto” part of the song. Also it works better if your accent is one which pronounces the word arse with the long a and the non-rhotic r, rather than the common North American pronunciation “asshole”.
2. Tell someone to shut the far cupboard. This works better if the person has previously opened a cupboard and neglected to shut it.