An extract from the Two Towers - The Choices of Master Samwise
At this point Sam began to listen more attentively and pressed his ear against the stone.
'Who cut the cords she'd put round him, Shagrat? Same one as cut the web. Didn't you see that? And who stuck a pin into Her Ladyship? Same one, I reckon. And where is he? Where is he, Shagrat? '
Shagrat made no reply.
`You may well put your thinking cap on, if you've got one. It's no laughing matter. No one, no one has ever stuck a pin in Shelob before, as you should know well enough. There's no grief in that; but think-there's someone loose hereabouts as is more dangerous than any other damned rebel that ever walked since the bad old times, since the Great Siege. Something has slipped.'
What is this "Great Siege" the two orcs are talking about?