Are there other threaded mechanical screws that are more like 'nipples'? Or did the end of spoked wheels at one point resemble nipples (say, for wooden wheels)? Why are they called 'nipples'?
In plumbing and piping, a nipple is a fitting, consisting of a short piece of pipe, usually provided with a male pipe thread at each end, for connecting two other fittings.
This is from the wikipedia entry on the plumbing and piping context of the word Nipple as suggested by Daniel R. Hicks' comment.
Can anyone elaborate on the etymology of this, or why this term for short pieces of pipe was used for spoke ends (perhaps proper piping nipples were used in the past)?
I would guess that the etymology is by analogy – the closest existing hardware to a spoke nipple was/is a pipe nipple so that when wheel makers started looking for a name for the little thing with the threads that tensions the spokes someone thought that they looked a bit like tiny pipe nipples and the name stuck.
From the Online Etymology Dictionary: Nyppell, dating from the 1530s, for a protuberance of a mammalian breast where the milk duct terminates in females, as an alteration of Neble from the 1520s, itself likely a diminutive of Neb (beak), making it a little beak, or little projection. The usage for small mechanical projections isn't attested before 1713, so the body part usage came first, while the usage for infant bottle teats didn't appear until 1875.
Nipple. It's just a fun word used to describe 'protrusion'. Something that 'pokes' or 'sticks' out. A 'nipple' was a nipple before it was a body part. I believe a woman's 'nipple' have been called many things before people got used to the fact it was just an average body part all humans have (men included). That said, to demystify the word "nipple" & bring it's truth to light, look around and see just how many things actually have a 'protrusion' of some sort and consider it might have been called a nipple before it was named something else. Like, an "outty belly button" ...isn't that just a stomach nipple?