Words that are not words

"I'm nauseous"

You would think this means I feel sick, but "nauseous" actually means to cause nausea. The correct phrase is "I'm nauseated."

"Irregardless"

As much as this is fun to say, the correct term is "regardless."

"Regardless of what you think, I am beautiful the way I look."

"Unhabitable"

Some place that is capable of being lived in is "habitable" or "inhabitable." Yes! Adding the "in" in front of "habitable" doesn't change the meaning!
If it is not, it is "uninhabitable."

"Runner-ups"

While the meaning is correct, someone finishing second place or behind the winner, the correct plural form is "runners-up," with the "s" coming after runner and not up.

"Themself"

The correct form of pronoun for a group of people is "themselves." Meanwhile, the singular forms are "himself" or "herself."

"Misunderestimated"

This is actually a double negative! "Mis" and "under" both negate "estimate." The correct term is "underestimate."

"Supposably"

This is a common misspelling of the word "supposedly." Be careful! The suffix is -edly not -ably.