Etymology
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interest (v.)
"cause to be interested, engage the attention of," c. 1600, earlier interesse (1560s), from the noun (see interest (n.)). Perhaps also from or influenced by interess'd, past participle of interesse.
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interest (n.)
mid-15c., "legal claim or right; a concern; a benefit, advantage, a being concerned or affected (advantageously)," from Old French interest "damage, loss, harm" (Modern French intérêt), from noun use of Latin interest "it is of importance, it makes a difference," third person singular present of interresse "to concern, make a difference, be of importance," literally "to be between," from inter "between" (see inter-) + esse "to be" (from PIE root *es- "to be"). The sense development to "profit, advantage" in French and English is not entirely clear.

The earlier Middle English word was interesse (late 14c.), from Anglo-French interesse "what one has a legal concern in," from Medieval Latin interesse "compensation for loss," noun use of Latin interresse (compare German Interesse, from the same Medieval Latin source).

Financial sense of "money paid for the use of money lent" (1520s) earlier was distinguished from usury (illegal under Church law) by being in reference to "compensation due from a defaulting debtor." Sense of "personal or selfish consideration" is from 1620s. Meaning "business in which several people are interested" is from 1670s. Meaning "curiosity, feeling that something concerns one, appreciative or sympathetic regard" is first attested 1771. Interest group is attested from 1907; interest rate by 1868.
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self-interest (n.)

also self interest, 1640s, from self- + interest (n.). Related: Self-interested.

[Self-interest] is a doctrine not very lofty, but clear and sure. It does not seek to attain great objects; but it attains those it aims for without too much effort. ... [It] does not produce great devotion; but it suggests little sacrifices each day; by itself it cannot make a man virtuous; but it forms a multitude of citizens who are regulated, temperate, moderate, farsighted, masters of themselves; and if it does not lead directly to virtue through the will, it brings them near to it insensibly through habits. [Alexis de Tocqueville, "Democracy in America"]
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disinterest (n.)

"impartiality," 1650s, from dis- "opposite of" + interest (n.).

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interested (adj.)
1660s, "characterized by concern or sympathy," past-participle adjective from interest (v.). From 1828 as "having an interest or stake (in something);" sense "motivated by self-interest" (1705) is perhaps a back-formation from disinterested. Related: Interestedness.
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interesting (adj.)
1711, "that concerns, important" (archaic), present-participle adjective from interest (v.). Meaning "engaging the attention, so as to excite interest" is from 1751. Related: Interestingly. Euphemistic phrase interesting condition, etc., "pregnant" is from 1748.
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uninterested (adj.)
1640s, "unbiased," from un- (1) "not" + past participle of interest (v.). It later meant "disinterested" (1660s); sense of "unconcerned, indifferent" is recorded from 1771. This is the correct word for what often is miscalled disinterested.
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*es- 

Proto-Indo-European root meaning "to be."

It forms all or part of: absence; absent; am; Bodhisattva; entity; essence; essential; essive; eu-; eucalyptus; Eucharist; Euclidean; Eudora; Eugene; eugenics; eulogy; Eunice; euphemism; euphoria; euthanasia; homoiousian; improve; interest; is; onto-; Parousia; present (adj.) "existing at the time;" present (n.2) "what is offered or given as a gift;" proud; quintessence; represent; satyagraha; sin; sooth; soothe; suttee; swastika; yes.

It is the hypothetical source of/evidence for its existence is provided by: Sanskrit asmi, Hittite eimi, Greek esti-, Latin est, Old Church Slavonic jesmi, Lithuanian esmi, Gothic imi, Old English eom, German ist.

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usury (n.)
c. 1300, "practice of lending money at interest," later, at excessive rates of interest, from Medieval Latin usuria, alteration of Latin usura "payment for the use of money, interest," literally "a usage, use, enjoyment," from usus, from stem of uti (see use (v.)). From mid-15c. as "premium paid for the use of money, interest," especially "exorbitant interest."
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usurer (n.)
late 13c., "one who lends money at interest," but later especially "one who lends money at an exorbitant rate of interest," from Anglo-French usurer, Old French usurier, usureor, from Medieval Latin usurarius "money-lender, usurer," from Latin usurarius (adj.) "pertaining to interest; that pays interest," from usura (see usury).
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