To be mordant is to be cutting, and perhaps sarcastic.
Here are a few things not to say unless you really must:
I am not young enough to know everything.
That's Oscar Wilde's line, bless him, and is useful for being cruel to children. Not that you should, obviously.
And then there's:
I have had a perfectly wonderful evening, but this wasn't it.
That's from the very great, but thankfully nowadays easily avoidable in person, Groucho Marx.
Or how about:
The covers of this book are too far apart.
Ouch! That's Ambrose Bierce.
Mordant can also mean pungent, but I wouldn't recommend being that, either.
A mordant can be either a substance that helps fix a dye, or one that etches lines on a metal plate, as well.
By the way, a mordent (with an e) is a very short trill-type thing in music. The definition in the Collins dictionary is slightly wrong, but then perhaps the Collins people have Van Gogh's ear for music.
That last mordant quotation is from Billy Wilder.
Thing Not To Do Today Unless You Really Must: be mordant. This word comes from the Old French, from mordre, to bite, from the Latin mordēre.