Upper Case and Lower Case - origin of the terms
There used to be two cases placed above the compositor. One case contained the capitals letters, which occupied the upper position so it is the "upper case" and one case contained the ordinary type so it is the "lower case." So the compositor had to press a lever to change from one case to the other.
The magazine I worked with at the start of my career would use this type of composing. The compositor used to confess that he won't live long as some of the lead would get into his system with constant handling. The result: lead poisoning! The maximum he expected to work was ten years. But he was happy with his job.
In those days people put their lives at risk working on these antiquated machines. Now we have computers to do the job, spelling and grammar checks, and autocorrect that corrects without our intervention. But still people are lazy and lethargic and prefer to consider themselves "geniuses" in the language who do not need editing.