Interest
late Middle English (originally as interess ): from Anglo-Norman French interesse, from Latin interesse ‘differ, be important’, from inter- ‘between’ + esse ‘be’. The -t was added partly by association with Old French interest ‘damage, loss’, apparently from Latin interest ‘it is important’. The original sense was ‘the possession of a share in or a right to something’; hence interest (sense 4 of the noun). interest (sense 1 of the noun) and the verb arose in the 18th century interest (sense 2 of the noun) was influenced by medieval Latin interesse ‘compensation for a debtor's defaulting’.
wiktionary
From Middle English interest, from Old French interesse and interest (French intérêt), from Medieval Latin interesse, from Latin interesse.
etymonline
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.
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.
