bandy (v.)

"an organized group," originally especially of armed men, late 15c., from French bande, which is traceable to the Proto-Germanic root of band (n.1), perhaps via a band of cloth worn as a mark of identification by a group of soldiers or others (compare Gothic bandwa "a sign"). But perhaps from Middle English band, bond in the sense "force that unites, bond, tie" (late 14c.). Also compare Old Norse band "cord that binds; act of binding," also "confederacy."

The extension to "group of musicians" is c. 1660, originally musicians attached to a regiment of the army and playing instruments which may be used while marching. To beat the band (1897) is to make enough noise to drown it out, hence to exceed everything. One-man band is by 1931 as "man who plays several musical instruments simultaneously;" figurative extension is by 1938.