early 15c., "of the first order," from Latin primarius "of the first rank, chief, principal, excellent," from primus "first" (see prime (adj.)). Meaning "first in order" is from 1802. Primary color is first recorded 1610s (at first the seven of the spectrum, later the three from which others can be made); primary school is 1802, from French école primaire.
The Paris journals ... are full of a plan, brought forward by Fourcroy, for the establishment of primary schools, which is not interesting to an English reader. [London "Times," April 27, 1802]