It not only means two opposite things (making it a contranym), but manages to mean something horrible in both of them.
Fearful means full of fear: fearful means frightening.
Fearful also has a meaning which falls in between its two opposite meanings. If you have a fearful cold then it's not a scary cold, nor an intimidating cold. It's just a severe and annoying cold.
To make things even more complicated we have the word fearfully, which, yes, can mean in a frightened way, but can also just mean extra. If you say I'm fearfully sorry it's nothing to do with being either afraid or annoyed.
(Mind you, this use of fearfully is fearfully out-of-date, so it's probably best avoided unless you are old enough to have used a wind-up gramophone and have fought in at least one World War.)
Anyway, fearful. Don't be it. Be brave. Be kind. Own up to your misdeeds. Stand up to the bullies.
It's not easy, but it's the price of a civilised world.
Thing Almost Certainly Not To Do Today: be fearful. This word is of course to do with fear, which comes from the Old English fǣr and is related to the Latin perīculum, danger.