I suppose I shouldn’t really call this an underrated or underused word, because it’s a French word, and since I don’t converse or read in French all the time, I have little-to-no idea how often it’s used.  I guess I should just consider this one of my favorite words in the French language.

un soupçon – Definition: suspicion; hint, drop (figurative)

David est à l’abri de tout soupçon – David is free of any suspicion.

Je sens juste un soupçon d’ail – I taste just a hint of garlic.

Veux-tu un soupçon de vin ? – Would you like a drop of wine?

See http://french.about.com/od/vocabulary/g/soupcon.htm (Mot du jour)

Also, see
Underrated, underused words, pt. 1