1297, from O.Fr.
rebelle (12c.), from L.
rebellis "insurgent, rebellious," from
rebellare "to rebel, wage war against," from
re- "opposite, against," or perhaps "again" +
bellare "wage war," from
bellum "war." The noun is attested from c.1400. Meaning "supporter of the American cause in the War of Independence" is from 1775; sense of "supporter of the Southern cause in the American Civil War" is attested from April 15, 1861.
"The Southern troops, when charging or to express their delight, always yell in a manner peculiar to themselves. ... The Confederate officers declare that the rebel yell has a particular merit, and always produces a salutary and useful effect upon their adversaries. A corps is sometimes spoken of as a 'good yelling regiment.' " [A.J.L. Fremantle, "Three Months in the Southern States," 1863]
The verb (1375) is from O.Fr.
rebeller, from L.
rebellare. Rebellion first attested c.1340;
rebellious is from 1432.
[n., adj. reb-uh
l; v. ri-bel] 
noun, adjective, verb, -belled, -bel⋅ling.