Yours, Etc.: Origins and Uses of 8 Sign-Offs
Old English bealu-full "dire, wicked, cruel," from bealu "harm, injury, ruin, evil, mischief, wickedness, a noxious thing," from Proto-Germanic *balwom (cf. Old Saxon balu, Old Frisian balu "evil," Old High German balo "destruction," Old Norse bol, Gothic balwjan "to torment"), from PIE root *bheleu- "to beat." During Anglo-Saxon times, the noun was in poetic use only (e.g. bealubenn "mortal wound," bealuðonc "evil thought"), and for long baleful was extinct, but it was revived by modern romantic poets. Related: Balefully.