(Latin). Something that is between the parties concerned. The interest of money is the sum which the borrower agrees to pay the lender for its use. To take an interest in anything is to feel there is something between it and you which may affect your pleasure.
Interest for money. In the Tudor dynasty it was 10 per cent. (37 Henry VIII. chap. 9). In the reign of James it was reduced to 8 per cent.; in Queen Anno's reign to 5 per cent.; in the last quarter of the nineteenth century it was reduced to 22 per cent.
Source: Dictionary of Phrase and Fable, E. Cobham Brewer, 1894