hado-

前缀

中缀

had-: Alternative form of hado- (“ocean depths”).
-o-: A linking vowel inserted interconsonantally between two morphemes, to ease pronunciation, without contributing to the meaning. It frequently joins words or combining forms of Ancient Greek or Classical Latin origin in the classical compounds of New Latin and international scientific vocabulary, but it can also be used to join modern terms and even abbreviations, either formally or informally.

From Learned borrowing from Ancient Greek ᾍδης (Hāídēs) + -o-. By surface analysis, had- +‎ -o-.