c. 1300, cumbren, combren, "to overthrow, destroy, probably a shortening of acombren "obstructing progress," from Old French encombrer, from combre "obstruction, barrier," from Vulgar Latin *comboros "that which is carried together," which is perhaps from a Gaulish word. The likely roots are PIE *kom (see com-) and *bher- (1) "to carry," also "to bear children."
Weakened sense of "to hamper in movement, to obstruct or weigh down" is from c. 1300. Related: Cumbered; cumbering. Cumber-world (late 14c.) was an old word for any thing or person that encumbers the world without being useful; cumber-ground (1650s) was "useless or unprofitable thing or person."