@Chinchy This bothered me so I looked it up.

"Guy" meaning "man" is from ~1850, a broadening of "guy" meaning "poorly dressed man" from ~1835, a shortened form of mockingly calling them "effigies of Guy Fawkes" in ~1805 (since those effigies were burned and therefore dressed in cheap/trash clothing). Fawkes tried to blow up the king and parliament in 1605, so the name predates the word by a lot.

(The name Guy derives from Guido, meaning "leader" and sharing etymological roots with "guide.")

And "girl" is derived from "gyrle" from the 1300s, meaning "child" (of either gender), which for unknown reasons shifted to mean "female child" and then "young unmarried lady" and then "any lady" over the course of ~500 years.