Answer:

boo

AnswerCrossword Clue
BOOCatcall
BOOGhostly greeting
BOOHalloween word
BOOScare word
BOOSerenade the villain
BOOBronx cheer
BOODoubled, the second part of a pint-sized beauty queen's name
BOO"Check your eyes, ump!"
BOOSack
BOOUnambiguous response to 51-Across's performance
BOOGoblin's greeting
BOOResponse to a bad call
BOOGreet the villain
BOOHalloween outcry
BOOGreeting from Casper
BOOWord to make you start
BOORebuke from a fan
BOOSpectral cry
BOOHeckling holler
BOOGhost word
BOORah's antithesis
BOOHalloween cry
BOOSurprising word
BOOArthur Radley's nickname
BOONickname for Arthur Radley
BOOScout's rescuer in "To Kill a Mockingbird"
BOOScout's neighbor, in "To Kill a Mockingbird"
BOONegative reaction
BOOVoice one's displeasure, in a way
BOOFan's rebuke
BOOHiss relative
BOO"You stink!"
BOOHeckler's holler
BOO"Eek!" preceder, maybe
BOODisapproving cry
BOOFan's complaint
BOOCrowd put-down
BOOHalloween greeting
BOOVerbalized displeasure
BOOFrightening sound