Etymology
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principal (adj.)

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"). 

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principal (n.)

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."  

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principally (adv.)

mid-14c., principalli, "mostly, mainly; most importantly," from principal (adj.) + -ly (2). Late 14c. as "first of all." Meaning "for the most part" attested by 1832.

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principality (n.)

c. 1300, principalite, "position of a prince," from Old French principalite "principal matter; power, sovereignty" (12c., Modern French principauté) and directly from Late Latin principalitatem (nominative principalitas), from principalis "first in importance; original, primitive" (see principal (adj.)).

Mid-14c. as "government by a prince." Meaning "region or state ruled by a prince or monarch" is attested from late 14c. Also in Middle English "state of being superior" (late 14c.).

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secondary (adj.)

late 14c., secondarie, "belonging to the second class; not chief or principal; second in importance or authority; of a lesser quality or worth; subordinate to something else, depending upon the action of primary qualities," from Old French secondaire and directly from Latin secundarius "pertaining to the second class, inferior," from secundus (see second (adj.)).

Opposed to primary (adj.) or principal (adj.). Of colors, under the old theory, from 1831; in reference to schools or education, from 1809. Of characteristics peculiar to one sex but not necessary for reproduction, from 1780. Related: Secondarily; secondariness.

As a noun from mid-15c. as "thing or place of secondary importance or which is dependent on a primary;" 1590s as "a delegate or deputy." The U.S. football sense of "defensive backfield" is by 1955.

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principle (n.)

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.

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*per- (1)

Proto-Indo-European root forming prepositions, etc., meaning "forward," and, by extension, "in front of, before, first, chief, toward, near, against," etc.

It forms all or part of: afford; approach; appropriate; approve; approximate; barbican; before; deprive; expropriate; far; first; for; for-; fore; fore-; forefather; foremost; former (adj.); forth; frame; frau; fret; Freya; fro; froward; from; furnish; furniture; further; galore; hysteron-proteron; impervious; improbity; impromptu; improve; palfrey; par (prep.); para- (1) "alongside, beyond; altered; contrary; irregular, abnormal;" paradise; pardon; paramount; paramour; parvenu; pellucid; per; per-; percent; percussion; perennial; perestroika; perfect; perfidy; perform; perfume; perfunctory; perhaps; peri-; perish; perjury; permanent; permeate; permit; pernicious; perpendicular; perpetual; perplex; persecute; persevere; perspective; perspire; persuasion; pertain; peruse; pervade; pervert; pierce; portray; postprandial; prae-; Prakrit; pre-; premier; presbyter; Presbyterian; preterite; pride; priest; primal; primary; primate; primavera; prime; primeval; primitive; primo; primogenitor; primogeniture; primordial; primus; prince; principal; principle; prior; pristine; private; privilege; privy; pro (n.2) "a consideration or argument in favor;" pro-; probably; probe; probity; problem; proceed; proclaim; prodigal; produce; profane; profess; profile; profit; profound; profuse; project; promise; prompt; prone; proof; proper; property; propinquity; prophet; prose; prostate; prosthesis; protagonist; Protean; protect; protein; Proterozoic; protest; proto-; protocol; proton; protoplasm; Protozoa; proud; prove; proverb; provide; provoke; prow; prowess; proximate; Purana; purchase; purdah; reciprocal; rapprochement; reproach; reprove; veneer.

It is the hypothetical source of/evidence for its existence is provided by: Sanskrit pari "around, about, through," parah "farther, remote, ulterior," pura "formerly, before," pra- "before, forward, forth;" Avestan pairi- "around," paro "before;" Hittite para "outside of," Greek peri "around, about, near, beyond," pera "across, beyond," paros "before," para "from beside, beyond," pro "before;" Latin pro "before, for, on behalf of, instead of," porro "forward," prae "before," per "through;" Old Church Slavonic pra-dedu "great-grandfather;" Russian pere- "through;" Lithuanian per "through;" Old Irish ire "farther," roar "enough;" Gothic faura "before," Old English fore (prep.) "before, in front of," (adv.) "before, previously," fram "forward, from," feor "to a great distance, long ago;" German vor "before, in front of;" Old Irish air- Gothic fair-, German ver-, Old English fer-, intensive prefixes.

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farm-house (n.)
also farmhouse, "principal dwelling-house of a farm," 1590s, from farm (n.) + house (n.).
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concerto (n.)

"composition for two or more solo instruments, or one principal instrument accompanied by a large or small orchestra," 1730, from Italian concerto (see concert (n.)). Concerto grosso is attested from 1724. Diminutive concertino is from 1857; earlier it meant "the principal instruments in a concerto or concertante" (1819).

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foreman (n.)
early 13c., "a leader," from fore- + man (n.). From 1530s as "principal juror;" 1570s in the sense of "principal workman." Similar formation in Dutch voorman, German Vormann, Danish formand. Also in 17c., a slang word for "penis." Fem. form forewoman is from 1709, originally of a jury; forelady is from 1867 in reference to juries, 1888 of shops, American English.
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