It really grinds my gears when people get the meaning of a word so wrong. Below are a number of words that I see people constantly misuse.

LACONIC: People use this word when referring to someone as being laid back when it actually means concise.

CONTINUAL: A lot of people think this word means with no interruption, when in reality it means occurring over a long period of time, not necessarily without interruption.

CHRONIC: People often use this word to describe a condition as being very bad, when in reality it means long-term.

DECIMATED: This word is often used by people when they are talking about something that has been destroyed when it actually means reduced by one-tenth.

PENULTIMATE: Far too often people assume this word means the ultimate when it really means second last.