One who, or that which, kicks. [1913 Webster]
A fact, condition, or circumstance, sometimes concealed or not obvious, which reduces or eliminates the benefit of an apparently advantageous situation; a joker{5}; as, under the Soviet system, bread was good and cheap, but the kicker was that you waited in line for hours to get any, if it was available. [PJC]
An unforeseen added expense or additional cost; as, the printer was cheap, but the special paper it needed was an expensive kicker. [PJC]

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THESAURUS

kicker

astonishment, bellyacher, blockbuster, blow, bomb, bombshell, boundary condition, catch, clause, complainant, complainer, condition, crab, crank, croaker, donnee, earthshaker, escalator clause, escape clause, escape hatch, eye-opener, faultfinder, fine print, frondeur, given, griper, grouch, grounds, grouser, growler, grumbler, joker, kvetch, limiting condition, malcontent, murmurer, mutterer, obligation, parameter, peripeteia, prerequisite, provision, provisions, proviso, querulous person, reactionary, reactionist, rebel, requisite, revelation, saving clause, shocker, sine qua non, small print, sorehead, sourpuss, specification, staggerer, startler, stipulation, string, surprisal, surprise, surprise ending, surprise package, surprise party, switch, terms, thunderbolt, thunderclap, ultimatum, whereas, whiner

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