mandatum

Mandatum, mandati, n., second declension, the neuter participle of mando, mandare, "hand over, entrust", turned into a noun. Mando is manus plus do, "give into the hand", so a mandatum is literally the thing put into someone's hands: a charge, an instruction.

Roman law made it precise. Mandatum is a named contract, an unpaid commission to act on another's behalf, and English mandate and mandatory come straight out of that. The Vulgate uses it as the standard word for a commandment, and mandatum novum do vobis at the Last Supper is where Maundy Thursday gets its English name, through Old French mandé. Do not tangle it with mando, mandere, mandi, mansum, "to chew", a third-conjugation verb that shares nothing with this one but its first two syllables.