A long, tedious
passage in a work such as a book or play: or any long
stretch of tedium
: or the quality of an author that causes such tedium, that is long-windedness
It is simply the French for length. Not appearing in Webster because it's a fairly recent borrowing into English. Not in the original OED, though when they did get around to listing it in the Supplement they gave it as first recorded in 1821.
I have used it, perhaps wrongly, perhaps in at best a cheeky way, in an opposite sense, for a long passage which is somehow fulfilling and impressive because of how little happens in a long period of time. Time pauses, actions cease to be important. The specific example I'm thinking of is the amazingly counterintuitive dead time from the very beginning of Sarah Kane's play Blasted; though I'm sure I could employ it with equal joy to passages in Beckett.