Old English bealu-full "dire, wicked, cruel," with -ful + bealu "harm, injury, ruin, evil, mischief, wickedness, a noxious thing," from Proto-Germanic *balwom (cognates: Old Saxon balu, Old Frisian balu "evil," Old High German balo "destruction," Old Norse bol, Gothic balwjan "to torment"), from PIE root *bhelu- "to harm." During Anglo-Saxon times, the noun was in poetic use only (in compounds such as bealubenn "mortal wound," bealuðonc "evil thought"), and for long baleful has belonged exclusively to poets. Related: Balefully.