E. Cobham Brewer 18101897. Dictionary of Phrase and Fable. 1898.
In Orders or In Holy Orders. Belonging to the clerical order or rank.
To take Orders. To become a clergyman.
The word order means not only a mandate, but also an official rank, and in the Catholic Church, a rule of life, as Ordo albus (white friars or Augustines), Ordo niger (black friars or Dominicans). In Holy Orders is in the plural number, because in the Protestant Church there are three ranks of clergymendeacons, priests, and bishops. In the Catholic Church there are four major orders and four minor ones. According to Du Cange, the Ordns majrs are Subdeacontus, Deacontus, Presbytertus, and Episcoplis (Subdeacon, Deacon, Priest, and Bishop).