What does the word tattarrattat mean?

The word "tattarrattat" doesn't have a real meaning in the conventional sense. It's a nonce word, created by the Irish writer James Joyce for his novel Ulysses.

It's meant to be a onomatopoeic representation of a knock on the door, specifically the sound of four distinct knocks, followed by a pause, and then two more knocks.

The sound is described in the novel as "four separate sharp raps" followed by a pause and then "two more distinct knocks".

So while it doesn't hold a specific meaning in the dictionary, it conveys the sound of a specific kind of knocking in Joyce's work.