[dan-robertson]

Manual means related to the hands. So manual labour is working with one’s hands or a manual transmission is operated with one’s hand. Also manuals are the keyboards on an organ where you put your hands (but don’t you pull out stops with your hands…)

However it is also easy to think of manual as being the opposite of automatic—something could be manually operated or a computer system might have manual overrides.

[dylan604]

Was a transmission referred to as manual transmission before automatic transmissions became a thing? I'd tend to think the transmission use of manual was specifically to designate it not being automatic.