I think replacing 'th' with 'f' is something (somefing? lol) akin to regional lazyspeak. It happens up here in Yorkshire as well as on Eastenders (lol) with people using 'f' instead of 'th' and dropping aitches (h), such as saying 'ouse' instead of 'house' (actually in Yorkshire we more often than not say 'are arse' instead of 'our house'). We also often shorten words. For example, words that end with 'ouldn't' (which are already contracted..lol) get shoartened even further to 'unt'. So 'shouldn't' become 'shunt'.
We all know that 'nothing' becomes 'nowt', but 'anything becomes 'owt' and 'something' becomes 'summat'.
Fun. isn't ('int') it?
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