Would we like to read Tim Bowen's explanation of the origins of this interesting word? Rather!
Rather is a rather odd word. The key to its different meanings and uses may be found in its origins, however. It is believed to be derived from an old adjective rathe, which meant ‘quick’. Rather was the comparative form and therefore meant ‘quicker’ or ‘more quickly’. It also had the meaning of 'earlier' or 'sooner' and it was a fairly short step from this to one of its modern-day uses to mean 'more willingly’, as in He doesn't want to learn - he'd rather stay at home and play video games. Interestingly, if your preference refers to another person, the past simple is used, e.g. I’d rather you didn’t mention this matter to anyone else. In the sense of preference, rather can also be used with than, as in They said they would rather die than abandon their homes.
A different use of rather is to mean 'to a fairly large degree', where its meaning is pretty much the same as quite, as in I realize I've been rather stupid and selfish or He left rather suddenly. Although historically a comparative itself, rather can also be used with other comparatives and here it cannot be substituted by quite but could be replaced by a bit, e.g. The problem is rather more complicated than we had expected. In the sense of a bit or a little, rather can also be used with adjectives in examples like It’s a good essay, but rather long and with too as in Toby had drunk rather too much.
An old-fashioned upper-class British English use of rather was as a positive response to an invitation: Would you like to come to dinner tomorrow? Rather!, where it simply means ‘yes’.