High-school juniors across the country, facing their first Preliminary SAT exams, are engrossed in improving their vocabulary. Here's a thought that might help: A word that means the opposite of another is an antonym; a word that looks as if it means one thing but means quite another could be called a phantonym, and warrants wariness. ¶ Phantonyms pop up in the usage of even so careful a speaker as President Obama. As William Safire noted in March, when the president said that he wanted the American people to have "a fulsome accounting" for his stimulus program, he meant full, whereas to punctilious authorities the word means disgusting, excessive, insincere. […] Likewise, noisome does not mean noisy but smelly, unhealthful. […] Enormity does not mean enormous but great wickedness, a monstrous act.
Source: wiktionary