Definition: the killing of a king
The “reg” comes from “regal.”
It’s unclear whether this specifically refers to assassination, coup, or execution. The Wikipedia page uses examples of all three.
Why I Looked It Up
In The LEGO Story:
The had been owned by a notorious figure […] who was convicted of regicide at Finderup Lade, stripped of his estate, and declared an outlaw.
Added on September 7, 2023
In 100 Things You Will Never Find:
In 1649, [Cromwell] because one the regicides – signatories to the order that saw King Charles I beheaded.