A person who works for a living. A wage earner of some sort.
Fred: I heard they're handing out $50k to people with bad mortgages. Do you think the average working stiff will see ever a dime??
Steve: No chance in hell.
A person sporting a woody at work.
Also, the action of working while having a stiffy.
I was working stiff this morning, bro. Still had a raging bone when I got to the office!