agelast

This word 'ultimately' and the infinitive ending on gelaein hide rather than illuminate the etymology. The root is gel- "laugh", with thematic ending -a- (this puts it into a subclass of verbs and shows up in many derivatives). Then gel-a-st- is an adjectival stem, showing up in gelast-os, -ê, -on "laughable" and the noun gelastês (feminine gelastria) "laugher, sneerer". With the negative prefix it is the adjective agelast-os, -ê, -on "unlaughing". It is this that Rabelais borrowed, dropping the ending as usual to fit it into French.

One who has imbibed water from the fountain of youth.

Mmm... oddly akin unto aghast.

Troopie, your source is showing!

From Greek agelastos (not laughing), ultimately from gelaein (to laugh).

Gr. agelastos, not laughing; ultimately from gelaein, to laugh. Coined by the French Renaissance writer Rabelais, or so the source quoted in the OED suggests.

Anyone have any sort of etymology for this?

One who never laughs.