Stiff - The Verb
SD has conjugated the verb still.
I stiff, you stiff, etc.
Unless you are using it in the slang to cheat someone I am sure how else you could use it as a verb.
3 Answers
This isn't a Spanish question, it's an English question - who has ever heard of the verb to stiff ?
To stiffen ........ Ponerse rígido algo
To stiff ....... Estafar a alguien (possibily "engañar" depending on context)
Stiff? It could be endurecer, fortalecer.
Used as slang it means engañar, trucar a alguien.