"To frighten" or "make fearful" is at the root of the verb intimidate.
An animal might intimidate a smaller animal by bearing its teeth, and a person can intimidate another by threatening to do something harmful.
When you intimidate, you frighten or make someone afraid.
A pet rat might intimidate your sister's friends, keeping them out of your fort.
Redford plays Depression-era confidence trickster Johnny Hooker, whose friend and mentor Luther Coleman (Robert Earl Jones) is murdered by racketeer/gambler Doyle Lonnegan (Robert Shaw).
Hoping to avenge Luther's death, Johnny begins planning a "sting" -- an elaborate scam -- to destroy Lonnegan.
But, so far from intimidating him, this only the more confirmed him in his convictions and fervency.