vex 
/ˈvɛks/ verb
vex·es; vexed; vex·ing /ˈvɛks/ verb [+ obj] old-fashioned : to annoy or worry (someone)
▪ This problem has vexed researchers for years. ▪ We were vexed by the delay.
▪ This problem has vexed researchers for years. ▪ We were vexed by the delay.
— vexing adjective [more vexing; most vexing]
▪ a vexing problem [=an irritating/annoying problem]
▪ a vexing problem [=an irritating/annoying problem]
