Meaning

adjective like or characteristic of or befitting a brother

adverb (archaic as adverb) in a brotherly manner

originFrom Middle English brotherly, from Old English brōþorlīċ, from Proto-West Germanic *brōþerlīk, from Proto-Germanic *brōþērlīkaz, equivalent to brother + -ly (adjectival suffix). Cognate with Dutch broederlijk (“brotherly”), German brüderlich (“brotherly”), Swedish broderlig (“brotherly”). Piecewise doublet of friarly.

Etymology from Wiktionary, CC BY-SA 3.0