TIL: The old English word for soldier was “cempa”, which survives in modern “champion” and the surname Kemp (Kemps were presumably soldiers the same way Smiths were blacksmiths). It was related to the German word “kampf” for battles (eg Mein Kampf, “My Battle”). Cempa and kampf both originally came from a similar proto-German word meaning “field”, related to Latin “campus” also meaning “field”, for a sense meaning battlefield, training field, etc - soldiers were people “in the field”, so to speak. The English word “camp” is vaguely related to this.

But during the late Roman Empire, Emperor Diocletian introduced a new coin, the solidus, so named because it felt big and solid. Legionaries got paid in solidi, so “solidi-getters” became slang for anyone in the military. This slang word spread across Europe and eventually made it into English as “soldier”.