Typically a "stiff" drink contains less mixers than alcohol. As a result a stiff drink is generally stronger, or stronger tasting.
A Sex on the Beach cocktail for example has 1 part peach schnapps, 2 parts cranberry juice, and 2 parts orange juice to only 1 part vodka. That isn't a stiff drink, but a shot of whiskey straight would be.
a mixed drink; booze and some other form of liquid like vanilla vodka and coke or hypnotic and monster
Stiff drinks are my favorite at parties.