O.E. byrðen "a load, weight, charge, duty;" also "a child;" from P.Gmc. *burthinjo "that which is borne" (cf. O.N. byrðr, O.S. burthinnia, Ger. bürde, Goth. baurþei), from PIE *bher- (1) "carry, give birth." The shift from -th- to -d- took place beginning 12c. (cf. murder). Archaic burthen is occasionally retained for the specific sense of "capacity of a ship." Burden of proof is recorded from 1590s.
"leading idea," 1640s, from earlier sense "refrain or chorus of a song," 1590s, originally "bass accompaniment to music" (late 14c.), from O.Fr. bordon "bumble-bee, drone," or directly from M.L. burdonom "drone, drone bass" (cf. Fr. bourdon, Sp. bordon, Port. bordão, It. bordone), of echoic origin.
(1.) A load of any kind (Ex. 23:5). (2.) A severe task (Ex. 2:11). (3.) A difficult duty, requiring effort (Ex. 18:22). (4.) A prophecy of a calamitous or disastrous nature (Isa. 13:1; 17:1; Hab. 1:1, etc.).