From Proto-Slavic *ključь, related to the verb *kljukati ("to hook, to bend"), referring to the hooked shape of early keys. Cognates include Polish klucz, Czech klíč, and Bulgarian ключ. The same root gives the verb включить ("to include/switch on") — literally "to hook in."

In Russian culture, handing over a key (especially to a new home) is a meaningful gesture of trust and belonging. The word ключ also means a natural spring of water in literary and poetic Russian — so you may encounter фонтан and ключ used differently depending on context. Watch out for the phrase под ключ ("turnkey / ready to move in"), common in real-estate advertisements.