This word is acceptable for play in the US & UK dictionaries that are being used in the following games:

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n. One that engages in aggression.

n. The person who first attacks or makes an aggression; he who begins hostility or a quarrel; an assailant.

n. The person who first attacks or makes an aggression; he who begins hostility or a quarrel; an assailant.

n. The person who first attacks; one who begins hostilities or makes encroachment; an assailant or invader.

n. someone who attacks

n. a confident assertive person who acts as instigator

"But if the aggressor is an SUV, the car driver's relative chance of dying rises to 30 to 1, because the hood of an SUV is so high off the ground."

instigator    initiator   

assailant   

Prof.    assessor    compressor    dresser    guesser    lesser    lessor    messer    oppressor    possessor   

assaulter    strop    troublemaker    aggressions    tried    when