The letter of the day is: NO!

Preferably said whilst hitting, kicking or writhing around as though committing fully to a grand mal seizure.

Mostly said in response to things like, "Sure hon, if you'd like a snack there are plenty of delicious things left in your lunchbox!" or "I'm happy to do that for you, just as soon as I finish this project that should only take five minutes but instead is taking 50 effing hours due to the incessant tantruming going on round here."

Most effective response to the letter of the day is not: BITE ME!
And yet, it does carry a certain satisfaction to say.
Not so much when you hear it regurgitated to a Kindergarten classmate or say, the kids father.

The letter of the day can be returned to the tantrum thrower, but somehow is less effective than stonewalling the kid.  And there's a sick satisfaction to giving your kid the silent treatment.  More so when you realize that nothing threatening to burst from your lips would be appropriate.

Once you calm down there is sometimes only one response to the child employing the word of the day in an over zealous matter: "I love you.  I think you're amazing.  But I will not cuddle you if you're going to hit/kick/pinch/yell at me.  Now go away."

Somehow this is less effective than one might hope.

You know what IS very effective?  This:

And now the letter of the day has become "Mahnahmahnah" and that is hella better than "NO!"

What's the letter of the day at your house?