Drop one's trousers

What is the meaning and origin of the phrase "drop one's trousers"?

I have only heard the literal meaning: if you went into a doctor's surgery (or a police station)and were told to "drop your trousers" you would undo your trousers and let them drop to the floor. Pamela

I hope this doesn't bring down the tone of the site, but a related phrase is "drop trou", which means to drop one's trousers (or pants as we tend to say in the U.S.). "Drop trou" is a slang expression that is associated with naughty goings-on rather than with a visit to the doctor's office.

Drop your linen and come a grinnin'. Or: Stop your grinnin' and drop your linen.