TH-fronting

TH-fronting refers to the pronunciation of "th" as /f/ or /v/. /θ/ becomes /f/ (so think sounds like fink) and /ð/ becomes /v/ (so mother sounds /'mʌvər/). It is a characteristic feature of London speech, particularly Cockney, i.e. what could be considered working-class speech, but also extends into much of South East England and, due to the influence of TV, as far afield as Glasgow.

The term fronting does not refer to the beginning (front) of the word, but to the front of the mouth. Fronting a /θ/ means that the teeth will be much forward (to the front) than normal. Examples of fronting at the end of words are when bath rhymes with laugh and when lithe rhymes with hive.