Origin of phrase "dead lumber?"

I think it might be a variation of the expression dead wood from the mill industry when nothing could be done with rotting or dead wood. In other words something or a situation with which nothing can be done. Stuck in Margate with no bus, a rail strike and hotels packed you're in dead lumber.

Hey! You're not old enough to drink! Now go and die for your country!!!

It's actually a mix of two expressions:

Lumber means trouble or a problem. You can be in lumber, which means that you're in trouble. Or you can be lumbered with means given a problem. For example, at a restaurant, you could be 'lumbered with the bill'.

Dead means very. As in, dead pretty means very pretty. Dead heavy means very heavy. Dead stupid means very stupid.

So, whilst it doesn't translate word for word, dead lumber just means a lot of trouble. As a Brit, I don't know of another Brit that wouldn't know what it meant without any sort of explanation.

I've no idea how 'lumber' came to mean 'trouble', but you have to be cautious when trying to draw a direct connection. In Cockney, there can be multiple levels of redirection between a word and the concept that it's used for. A good example of this is in To Hull And Back: when Rodney is objecting to being a decoy for Customs when Del is planning on bringing the diamonds back on a plane, he says something like, "That's great. Whilst you're off down the kermit, it's touch your toes time for Rodney." Here 'kermit' means road, but it's a riff on the rhyming slanging for 'road'. You have to to go: Kermit => Kermit the Frog => Frog & Toad => Road.