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set (someone or something) (up)on (one)
To command, instruct, or order someone or an animal to attack someone. The guards set the dogs on the would-be thieves. The crime boss set his goons upon the accountant who refused to launder money for them.
set (up)on (doing something)
Determined to or focused on doing something. My cousin, set upon being the best tennis player in the world, trains for three hours every day. She's been set on becoming a fighter pilot ever since she was a little girl.
set (up)on (someone or something)
To viciously and violently attack or maim someone or something. The pair of thieves set upon the traveling merchant, stealing his goods and leaving him half-dead on the side of the road. The parade descended into chaos as a drunken band of teenagers set upon the main float and began tearing it to pieces. The deer was set on by the mountain lion.
set upon someone or something
to attack someone or something violently. The dogs set upon the bear and chased it up a tree. Bill set upon Tom and struck him hard in the face.
To attack someone or something violently: The gang set upon their victim with clubs.