burden: There are two distinct words burden in English. By far the older, ‘load’, comes from Old English byrthen. Like bear, birth, bairn, bier, barrow, and berth it goes back ultimately to an Indo-European base *bher-, which signified both ‘carry’ and ‘give birth’. Its immediate Germanic ancestor was *burthi-, which also gave German bürde ‘load’. The other burden, ‘refrain’, and hence ‘main theme’, is an alteration of an earlier bourdon , which was borrowed from Old French bourdon ‘bass pipe’. => bairn, barrow, bear, berth, bier, birth
"a load," Old English byrðen "a load, weight, charge, duty;" also "a child;" from Proto-Germanic *burthinjo- "that which is borne" (cognates: Old Norse byrðr, Old Saxon burthinnia, German bürde, Gothic baurþei), from PIE root *bher- (1) "to bear, to carry; give birth" (see infer).
The shift from -th- to -d- took place beginning 12c. (compare murder (n.), rudder). Archaic burthen is occasionally retained for the specific sense of "capacity of a ship." Burden of proof is recorded from 1590s.
"leading idea," 1640s, a figurative use from earlier sense "refrain or chorus of a song," 1590s, originally "bass accompaniment to music" (late 14c.), from Old French bordon "bumble-bee, drone," or directly from Medieval Latin burdonom "drone, drone bass" (source of French bourdon, Spanish bordon, Portuguese bordão, Italian bordone), of echoic origin.