Learn English – What’s the origin behind the phrase “assume room temperature” which means “to die”

Based on a first glance, it's looking like the original expression is "have a monkey on the roof," meaning a mortgage, dating back to the 1800s. Later "on the back" forms referred to any unpleasant burden, and in the 1940s began to be applied to narcotics addiction. No citations worth posting here, so the above is speculative.

Deep in/into the weeds, also in deep weeds: is an idiomatic expression which has become very popular in recent years:

• (Of a restaurant worker) overwhelmed and falling behind in serving customers: Our waitress was so deep in the weeds that we waited 40 minutes for our burgers.

• In trouble; overwhelmed by problems: He knows our marriage is in deep weeds.

• Involved in the details: I’m in the weeds of planning my wedding.

According to The word detective:

• The restaurant term Is invoked when the staff is overworked, everything is going wrong, and total chaos is only a burnt fillet of sole away. Back in 2000 there was actually a Molly Ringwald movie about the staff of a restaurant dealing with a bad night called “In the Weeds".

regarding getting overwhelmed:

• My initial suspicion about “into the weeds” was that it had something to do with a golf ball landing in the “rough” (long grass), making it hard to extract without falling behind. I tend to think that golf is indeed the source of this sense of “getting into the weeds” meaning “losing control and being overwhelmed.” Other possible sources that have been suggested include a swimmer becoming tangled in seaweed and a boat having its propeller snarled by weeds in a lake.

and 'into details':

• But as Mark Liberman points out, the use of “into the weeds” to mean “delving deep into the details” doesn’t carry the same sense of painful confusion as the restaurant use, and such “weed wandering” is actually the sort of thing true policy wonks enjoy. As he says in his Language Log post:

  • “The metaphor here seems to be that when you wander off the beaten path, you can explore arbitrary amounts of not-very-valuable intellectual foliage (“weeds”) without getting closer to your conceptual destination.” I think that image of “wandering off the beaten path to examine interesting details along the way” is the key to this sense of “getting into the weeds.”

• Of course, some tasks actually require “getting into the weeds,” dealing with small but important details, such as the minutiae of financial or legislative analysis (“A panel of lawmakers is starting to ‘get into the weeds,’ as one state senator put it, and are hoping to write first drafts of possible new laws by the end of the summer addressing Montana’s wide-open medical marijuana scene,” Missoulian, 6/28/10).

• This sense of “getting into the weeds” would thus lie midway between the terror of a bad night as a waiter and the policy wonk’s eager embrace of statistical trends. Sometimes wandering around in the weeds is just all in a day’s work.