Hi Rocky,
A"stiff" jab refers to a hard (as opposed to a pitty-pat) piston-like, "spearing" jab that has head snapping power behind it. Go to YouTube and do a search on a prime Larry Holmes to see several examples. In my opinion, he had the best jab in heavyweight history. I'd rank the 60's Muhammad Ali as having the second best ever. Check him out too...

Take Care,
Lito