A nub, or nubbing, is a passage of nonsense that sounds like Shakespeare. Formerly any Shakespearian actor would have nubs securely memorized, so that they could slip into them effortlessly when they dried up. They contained cues for other actors to warn them what had happened, and when the nub was about to run out. Either another actor could somehow take over, prompt them, or skip ahead, or the dry actor could get to the prompter's box, as the speech of some half dozen or so lines went on.
Near the beginning and end of the passage the word 'nub' itself occurred, or 'nubbing', and by convention the very last words of the nub were 'Milford Haven'. This is the port in Wales where Henry VII landed en route to seize the throne, and fits in well in one of Shakespeare's history plays, though less happily in the others.
It must of course make sense in a vague way, because the audience are listening: it can't literally be nonsense; but it can't afford to say anything definite. In particular, it can't affect the plot, and it mustn't matter where in the plot they are. So nubs are composed of all the kinds of language that make least sense when you do Shakespeare in school, where you know you're probably missing puns or allusions or jokes but can't tell what.
This was back in the gaslight days: the house was fully lit, and the actors could see the audience's reaction. They knew which bits made them glaze over, and one of the most glazing things is the verbal quibble. I'm going to make up a Shakespearian quibble now, impromptu:
And for that a word, were it called a word,
Should unword those that call it, e'en as we do call anew;
Lest, no words left to call, our very selves should seek
What we did not seek ere now, yet e'er call that word a word.
That sort of thing. Nubbing is in some ways similar to greeking, the insertion of that Lorem ipsum passage into text for test or layout purposes; but greeking isn't intended to deceive. Nubbing is not just rhubarb, background chatter, but a tried and true strategy to get over awkward situations without alerting the audience. Each actor had their own nubs, because if a play was going badly (like if they were all pissed, heaven forfend) they might all have to use them, and it could get pretty obvious if they said the same things.
I heard of this practice just now on the BBC literary radio programme The Verb. They read out three examples. When I went to google for some to use in this node, I couldn't find any mention of the practice at all. But I'm going to trust that it wasn't a spoof. --- Well, it wasn't: there is now (March 2005) one other example on the Web, an article from The New Statesman, at: http://www.newstatesman.com/200503280033