In colloquial British English, "gaffer" means a foreman, and is used as a synonym for "boss". In the UK, the term also commonly refers to sports coaches (football, rugby, etc.).
The term is also sometimes used colloquially to refer to an old man, an elderly rustic. The word is probably a shortening of "godfather", with "ga" from association with "grandfather".
The female equivalent, "gammer", came to refer colloquially to an old lady or to a gossip. The use of gaffer in this way can be seen, for example, in J.R.R. Tolkien's character Gaffer Gamgee.