From Middle English beste, from Old English betst, betest (“best”), from Proto-Germanic *batistaz (“best”), from Proto-Indo-European *bhAd- (“good”). Cognate with Scots best (“best”), West Frisian best (“best”), Dutch best (“best”), Low German best (“best”), German beste (“best”), Danish bedst (“best”), Swedish bäst (“best”), Icelandic bestur (“best”). More at better.
From Wiktionary
From German Beste (“a river in Germany”) or from Anglo-Norman beste (“beast”).
From Wiktionary
Middle English from Old English betst bhad- in Indo-European roots
From American Heritage Dictionary of the English Language, 5th Edition