baleful (adj.)

Old English bealufull "dire, wicked, cruel," with -ful + bealu "harm, injury, ruin, evil, mischief, wickedness, a noxious thing," from Proto-Germanic *balu- (source also of Old Saxon balu, Old Frisian balu "evil," Old High German balo "destruction," Old Norse bol, Gothic balwjan "to torment"), a word of uncertain etymology.

During Anglo-Saxon times, the noun was in poetic use only (in compounds such as bealubenn "mortal wound," bealuðonc "evil thought"). The equivalent noun is missing in modern German, Danish, and Swedish, and in English bale long has been archaic or poetic only (OED says "Marked obsolete in dictionaries soon after 1600"), while baleful in modern English long has been poetic or literary only. Related: Balefully.