Английский Этимологический словарь - principal (adj.)

- c.1300, from L. principalis "first in importance," from princeps (see prince). The noun is c.1300 in the sense of "ruler;" c.1430 in the sense of "money on which interest is paid;" 1827 as "person in charge of a public school," though meaning "head of a college or hall" was in England from 1438.