Yours, Etc.: Origins and Uses of 8 Sign-Offs
"military attendant who carries orders," 1781, short for orderly corporal, etc. Extended 1809 to an attendant at a hospital (originally a military hospital) charged with keeping things in order and clean. See orderly (adj.).
orderly or·der·ly (ôr'dər-lē)
An attendant in a hospital.