Borrowed from Old East Slavic холопъ (xolopŭ, “serf, slave”) (cf. Russian холоп (xolop)), itself a borrowing from Turkic (or, according to some researchers, a native word, ultimately from Proto-Indo-European *kel-, *skel- “offshoot, seedlings, stem”). The borrowing must have happened by the 13th century at the latest; the first mentions of this word are in 17th-century dictionaries.