Is 야하다 a pure-Korean word?

ANSWER: No, the 야 in 야하다 is the Chinese character 冶 (야), which besides meaning "to smelt" or "to cast," can also mean "to be seductive in appearance." I don't know why the definition in my Korean-English dictionary uses "gaudy" instead of "seductive."

And here is an example of 冶 (야) being used with the meaning "to smelt" or "to cast."

Finally, 冶容 (야용) can literally translate as either "a seductive (冶) face (容)" or "to cast (冶) a face (容)," which means to put on makeup to make one's face more seductive. Korean "kisaengs" (기생 妓生) apparently used to use makeup, and 황진이 was one of the most famous.