It comes from the Proto-Slavic word *milъ (sweet, dear), from the Proto-Indo-European word *meh₁y- (mild, soft).
The Czech word milý (nice, kind, good, dear, pleasant, sweet; boyfriend) comes from the same root, as do similar words in other Slavic languages, such as the Belarusian мілы (sweet, nice), the Bulgarian мил (dear), and the Polish miły (nice, pleasant).
The Latin mītis (gentle, mild, ripe) comes from the same Proto-Indo-European root, as does the Italian word mite (mild, moderate, balmy), the Portuguese word mitigar (to mitigate), the Spanish word mitigar (to mitigate, alleviate, allay, assuage, quench, soothe), and the English word mitigate.
I’m would like to put together pages of terms of endearment / affection in other languages. Can you help with this?