Refers to the academic discipline that investigates the origins and historical development of words.
彼は大学で語源学を専攻している。
He is majoring in etymology at university.
With knowledge of etymology, you can understand the history of words well.
Compound of 語源 (word origin) + 学 (study, -ology). The exact historical derivation is uncertain, but the term follows a common pattern for academic disciplines.