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).
@Connor 🧠 🧠 🧠 🧠 : the Chinese word for honey is an Indo-European borrowing via Tocharian, cognate with English mead.