Submitted via IRC for SoyCow1984
Horizontally-opposed engines are often referred to as “boxer” engines because the motion of the pistons sort of resembles a boxer, punching. Really, though, if any engine should be named for a boxer, it’s the other kind of opposed engine: the kind where the pistons actually “punch” right at each other. They’re called opposed-piston engines, and they’re fascinating.
Source: https://jalopnik.com/its-time-to-learn-about-wonderful-and-weird-opposed-pis-1827804895 [jalopnik.com]