From Latin 'interesse' (to be between, to matter) — the financial sense was a medieval euphemism to avoid the sin of 'usury.'
The feeling of wanting to know or learn about something; also, money paid regularly at a particular rate for the use of money lent, or for delaying the repayment of a debt.
From Latin 'interest' (it matters, it concerns, it makes a difference), the third person singular of 'interesse' (to be between, to be among, to concern, to matter), from 'inter-' (between, among) + 'esse' (to be), from PIE *h₁es- (to be). The original sense was 'being between' or 'being involved in' — what is between you and a thing is your interest in it. The financial sense arose from Medieval Latin: 'interesse' was used for the compensation due for loss or damage caused by a late payment
The financial sense of 'interest' arose from a legal workaround. Medieval Christian law forbade 'usury' (charging for a loan), but permitted 'interesse' — compensation for the damage a lender suffered from not having their money. Technically, you were not paying to borrow money; you were compensating for the lender's loss. The euphemism