Jacob Grimm, in his Teutonic Mythology (ch. 11), identifies Old Norse Baldr with the Old High German Baldere (2nd Merseburg Charm, Thuringia), Palter (theonym, Bavaria), Paltar (personal name) and with Old English bealdor, baldor "lord, prince, king" (used always with a genitive plural, as in gumena baldor "lord of men", wigena baldor "lord of warriors", et cetera). Old Norse shows this usage of the word as an honorific in a few cases, as in baldur î brynju (Sæm. 272b) and herbaldr (Sæm. 218b), epithets of heroes in general.
Grimm traces the etymology of the name to *balþaz, whence Gothic balþs, Old English bald, Old High German pald, all meaning "bold, brave".
But the interpretation of Baldr as "the brave god" may be secondary. Baltic (cf. Lithuanian baltas, Latvian balts) has a word meaning "the white, the good", and Grimm speculates that the name may originate as a Baltic loan into Proto-Germanic.
In continental Saxon and Anglo-Saxon tradition, the son of Woden is called not Bealdor but Baldag (Saxon) and Bældæg, Beldeg (Anglo-Saxon), which shows association with "day", possibly with Day personified as a deity. This, as Grimm points out, would agree with the meaning "shining one, white one, a god" derived from the meaning of Baltic baltas, further adducing Slavic Belobog and German Berhta.
Grimm's etymology is endorsed by modern research. According to Rudolf Simek, the original name for Baldr must be understood as 'shining day'.