A confession: until very recently I thought a spendthrift was someone who was careful (thrifty) with money.
In reality, as I'm sure you know, a spendthrift is the very opposite - "one who spends money profusely or wastefully; one who wastes his patrimony by foolish or lavish expenditure; an improvident or extravagantly wasteful person" (OED Online).
World Wide Words: Spendthrift
What words have you misunderstood (or if you prefer, reinterpreted)?
1 day ago