Middle English, "evil-doing, threat of evil, harm, destruction of life, torment, grief," going back to Old English bealu (neuter), genitive bealwes "evil-doing, malice, harm, destruction, torment," going back to Germanic *balwa- "evil," in both active and passive aspects (whence also Old Frisian bale- [in balemunda "disloyal guardian"], Old Saxon balu "wickedness," Middle Dutch bal- [in baldadich "wanton"], Old High German balo, palo "evil, disaster, treachery," Old Icelandic bǫl "misfortune," Gothic balwa- [in balwawesei "wickedness"]), noun derivative from neuter of an adjective *balwa- (whence Old English bealu "evil, pernicious"), going back to dialectal Indo-European *bhol-, whence also Old Church Slavic bolĭ "sick person," Bosnian-Croatian-Serbian bôl "pain," Russian bol'
Note: The etymon *bhol- is attested with certainty only in Germanic and Slavic, assuming that this comparison itself is correct. Proposed links with other Indo-European branches, as Celtic, are less convincing.
Noun (2)
Middle English, from Middle French, of Germanic origin; akin to Old High German balla ball