indefatigable

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indefatigable [in-di-FAT-i-guh-buhl]
adjective

definition
showing sustained enthusiastic action with unflagging vitality

etymology
Middle French, from Latin indefatigabilis, from in- ("not") + defatigare ("to fatigue"), from de- + fatigare ("to fatigue")

usage
Arnold had thought maybe today would be different, but twelve spitballs in his hair and a bowl of tapioca pudding down his shirt (and it wasn't even recess yet) proved that Helga would be indefatigable in her bullying, even on his birthday.