Normally it means there is less mixers than alcohol. A Virgin Mary (A Bloody Mary with no Alcohol) would not be stiff and a shot of whiskey would be stiff; as a comparison.
In slang, "stiff" can mean boring, difficult, or unyielding. It is often used to describe someone or something that is uninteresting or inflexible.
The cast of A Stiff Drink - 1996 includes: Ruth Gemmell as Woman Cal Macaninch as Man
Soda?
stiff
To get a Stiff, or Stiffy, is school playground giggles for an erection
To jump with stiff legs means to lock your knees.
Stiff
When a chronic alcho has a stiff drink he will stop "shaken"
Short,stiff hair
stiff or made out of wood
Short,stiff hair
If you mean the foreskin getting stiff, you need to clean your head more and maybe start gently masturbating If you mean your whole penis getting stiff it is because of the blood rushing to it when you become sexually aroused...