TIL the usual Slavic words for "king" (e.g. Russian король, Polish król) come from a borrowing into Proto-Slavic of the name "Karl," as in Charlemagne.
Has to be proto-Slavic because the cognates show a standard sound correspondence in which vowel+liquid becomes liquid+vowel (in West and South Slavic) or vowel+liquid+duplicate vowel (in East Slavic) - see also Pol. mleko and Russian молоко "milk" (both from a Germanic borrowing), or the Russian cities with names in -град (Old Church Slavonic borrowing) or -город (native East Slavic form).
Has to be proto-Slavic because the cognates show a standard sound correspondence in which vowel+liquid becomes liquid+vowel (in West and South Slavic) or vowel+liquid+duplicate vowel (in East Slavic) - see also Pol. mleko and Russian молоко "milk" (both from a Germanic borrowing), or the Russian cities with names in -град (Old Church Slavonic borrowing) or -город (native East Slavic form).