“I borrowed ten dollars and paid back eleven — the extra dollar was called interest.”
From Latin interest, which meant “it matters, it makes a difference”. The money sense grew out of that: what you pay because the loan matters to somebody.
Catch “interest” used both ways today — once about money, once about curiosity. Say which one you heard first.
💬 If a friend hands you back $22 on a $20 loan, is the extra $2 interest — or the start of a different kind of friendship?