Pronounced AY-jel-ast, this five dollar word is an adjective used to describe someone who never laughs; it's sometimes used to refer to a person who can't get any joy or happiness out of life. It's derived from the Greek agelastos ("not laughing"), itself stemming from gelaein ("to laugh").
Note: Gritchka believes agelast was coined by Rabelais, something I have been unable to confirm. On the basis of this and several other incidents, however, I can confirm that Gritchka is more learned than I. (As if anyone ever doubted that.)