Etymology

custos regni

Today it means: A kind of viceregal locum tenens appointed, either alone or jointly, to execute the political duties of a monarch during his or her absence from the seat of government. Full definition →

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The story

First attested in the plural in October 1644 and in the singular in 1683; Latin: custōs (“guard”, “protector”; “guardian”; “keeper”, “custodian”) + rēgnī (“of royal power”; genitive singular of rēgnum, “royal power”, “kingdom”) = “custodian of royal power” ≈ “guardian of the realm”.

Etymology from Wiktionary, CC BY-SA 4.0.