"officer or servant who purchases provisions for a college, monastery, etc.," c. 1200, from Old French manciple "steward, purveyor," from Medieval Latin mancipium "office or function of a manceps," from manceps (genitive mancipis) "a purchaser, contractor," etymologically "a taking in hand," from manus "hand" (from PIE root *man- (2) "hand") + stem of capere "to take" (from PIE root *kap- "to grasp"). In classical Latin, mancipium was "a servant, slave, slave obtained by legal transfer" (compare emancipate); also "a formal purchase, the legal purchase of a thing."
late 14c., "origin, source, beginning" (a sense now obsolete), also "rule of conduct; axiom, basic assumption; elemental aspect of a craft or discipline," from Anglo-French principle, Old French principe "origin, cause, principle," from Latin principium (plural principia) "a beginning, commencement, origin, first part," in plural "foundation, elements," 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").
The English -l- apparently is by analogy of participle, manciple, etc., also principal. From the notion of "one of the fundamental tenets or doctrines of a system, a law or truth on which others are founded" comes the sense of "a right rule of conduct" (1530s).
It is often easier to fight for principles than to live up to them. [Adlai Stevenson, speech, New York City, Aug. 27, 1952]
Scientific sense of "general law of nature," by virtue of which a machine or instrument operates, is recorded from 1802.