c. 1300, "main, principal, chief, dominant, largest, greatest, most important;" also "great, large," from Old French principal "main, most important," of persons, "princely, high-ranking" (11c.) and directly from Latin principalis "first in importance; original, primitive," from princeps (genitive principis) "first man, chief leader; ruler, sovereign," noun use of adjective meaning "that takes first," from primus "first" (see prime (adj.)) + root of capere "to take" (from PIE root *kap- "to grasp").
c. 1300, "chief man, leading representative," also "the most part, the main part;" also, in law, "one who takes a leading part or is primarily concerned in an action or proceeding;" from principal (adj.) or from or influenced by noun uses in Old French and Latin.
From mid-14c. as "ruler, governor;" 1827 as "person in charge of a public school," though the meaning "head of a college or hall" was in English from mid-15c. From early 15c. as "a main sum of money," hence "money on which interest is paid."