I've come across the answer about what it means to bleed something but am having a hard time finding its origin. What was the original thing that was bled and in what context was it used?
Cambridge Dictionary of American Idioms explain this
bleed somebody/something dry
to use up everything someone or something has available
The city is losing money at a rate that eventually will bleed it dry. I'm worried that the medical bills will bleed my parents dry.
Etymology: based on the idea of a person losing so much blood that they die
This fits with the usage of "bleed something" as a negative and damaging one, and not therapeutic. This is pretty much what @Robusto's answer to your related question states.
There seems to be a good deal of confusion in the answers here, and in the answers to the older question referenced by this questioner between at least two different ways "to bleed something":
[actor causing bleeding] bleeds [object from which something flows]
- A 17th century physician bleeds the patient to cure the disease.
- Last, the plumber bleeds the pipes.
- Losing money will bleed the city dry.
[subject from which something flows] bleeds [material which flows]
- Detroit bleeds manufacturing jobs.
The original question was about the latter kind of usage, and as best I can tell, this question is about that also.
So, the metaphorical use of to bleed seems to stretch into the distant past. In the second kind of usage, the earliest example the Oxford English Dictionary gives is this line from Shakespeare's Winter's Tale (V. ii. 88):
Shee did (with an Alas) I would faine say, bleed Teares; for I am sure, my heart wept blood.
I would say that it refers to the practice of blood letting using leaches. Physicians of days past, I need to find an exact time-dated reference for you, would bleed patients of "harmful humours". The idea was that blood loss would cure the patient of his ailment.
Summary, w/specific answers to your question:
What was the original thing that was bled? A human being.
In what context was it used? By medical practitioners (and the general public) of the 14th-19th centuries, in reference to a healer/ physician method for treating illness.
Side note: This also why physicians themselves were referred to as leaches, but not in a pejorative way, i.e. connoting fee-gouging behavior.