To talk on and on about nothing much. The person is not lying and not being rude — they are just filling the air, and the listener has stopped caring. It almost always comes with on: "he wittered on about his garden for an hour." You can also witter away. What you witter about takes about: "wittering on about the weather". This is British and Irish English. Americans do not really use it; they would say "ramble on" or "go on and on". It is informal, and it is mildly unkind — you would say it about someone, not to them, and you would use it about yourself as a joke: "sorry, I'm wittering." Notice the size of the complaint: it is small. If someone is being boring and self-important, English has harder words. Witter is closer to fond irritation, the way you might describe an uncle at a family dinner. There is also a noun use, witterings, meaning someone's not-very-important talk or writing — often used by people about their own blog or notes, as a way of being modest.