Circumspect. I thought this word meant that something was shady or something that shouldn’t be trusted. Today I found out that it means cautious, prudent or discreet.
What are some words that you thought you knew the meaning of until very recently?
Coven and convent.
I’ve had them confused for years.
For a long time I thought a Concussion was just another word for a cut.
enervate. It sounds like it means “to energize”, but really means just the opposite.
"to deprive of force or strength; destroy the vigor of; weaken. "
J.
Lugubrious. I thought it meant animated and chatty- maybe I was thinking of loquacious.
Spendthrift–I used to think it meant thrifty, but it means the exact opposite.
Scatological. I “know” what it means, but to me it means something like Scatterbrained.
I have probably used it that way before. Ouch.
When I was very young, there was a sign at the grocery store across the street that said “Shoplifters will be prosecuted”. I thought that “prosecuted” was some kind of fancy word for “executed”, and wondered how the store was allowed to get away with such a strict punishment…
I always thought that chartreuse was a kind of red.
Until last year the only context I’d ever heard the words Decadence and Decadent are food-related, and it always suggests richness. So imagine my surprise to find out that Decadance is a bad thing, meaning decay.
RadicalPi, for some uknown reason *a lot *of people mix up fuchsia (vivid purplely-pink) and chartreuse (sickly greenish yellow).
My guess is that people confuse “chartreuse” (the color of a particular liqueur) with the vaguely similar sounding “cerise” (the fuchsiaesque color of cherries).
Fuchsia is purple? What’s the bluish one I’m thinking of then?
Did you hear about the fashion designer who came up with a “Sari/Sarong number”? 
I was in my mid-20s when I discovered that “prostate” and “prostrate” were two different words.
Periwinkle?
Yep, it was a uniform for Indian call-centre staff. 
I used to think this too; I suspect that, because “watchfulness” is one aspect of its meaning, people extrapolate that to those that are being shady or untrustworthy being careful not to be detected.
Cyan.
This is me. Everytime I see Chartreuse I have to think about it.
I admit, until this thread I was thinking of Chartreuse being reddish too.