There's something sinister about the word mirth.
Mirth is the grin of a white-faced clown which hides who-knows-what desire for disaster and mayhem.
Mirth is the echo of cruel laughter...
...actually, at this point I'm forced to wonder if my problems with the word mirth are entirely my own; but how about Edith Wharton's book The House of Mirth, which will, I fear, disappoint those looking for a comedy; and how about The Mirth Corporation, whose mission statement, Powering Healthcare Interoperability, suggests that the waters may be even darker than I'd thought.
Hey, and do you know about the USS Mirth? She was a mine-sweeper.
Her eventual fate is unknown...
Word Not To Use Today: mirth. This word comes from the Old English word myrgth, which doesn't sound much fun, either.