Why does `buck` mean `step-down`?
It's the same sense as to "buck" a trend:
- to oppose or resist (something that seems oppressive or inevitable). "the shares bucked the market trend" synonyms: resist, oppose, contradict, defy, fight (against), go against, kick against "it takes guts to buck the system"
So you're "bucking" the input voltage to reduce the output voltage.
I may be wrong(apparently there is no way to qualify any answer here as correct), but I had always assumed that "buck" referred to an action similar to a "bucking bronco or bull". A buck converter sends a voltage pulse only as often as it needs to in order to provide the rectified and filtered DC output required, just as a "bucking bronco or bull" will "buck" as often as he feels he needs to in order to eject the rider or loosen the strap.
Question probably belongs on english.stackexchange.com. It arises from bucking being an action an animal takes to throw riders off or down, so a buck converter "throws" the voltage down by a repetitive "bucking" mechanism.
buck (v.1)
of a horse, "make a violent back-arched leap in an effort to throw off a rider," 1848, apparently "jump like a buck," from buck (n.1). Related: Bucked; bucking. Buck up "cheer up" is from 1844, probably from the noun in the "man" sense.
(from etymonline.com)