verb

surprise (someone) greatly; astonish.

news that activists could be fined for protesting against the policy will flabbergast most people

astonish,astound,amaze,surprise,startle,shock,take aback,take by surprise,dumbfound,strike dumb,render speechless,stun,stagger,stop someone in their tracks,take someone’s breath away,confound,daze,overcome,overwhelm,nonplus,stupefy,disconcert,unsettle,bewilder,bowl over,knock for six,knock sideways,knock the stuffing out of,floor,gobsmack