EIDIOL.  From the Welsh vocabulary word eidiog/eidiol, meaning "lively, vigorous, wanton." 

Eidiog, Eidiol a, lively, vigorous. (An English-Welsh Pronouncing Dictionary, Spurrell, 1872).

Eidiog, a. the same as Bywiog, and Nwyfus, lusty, lively, mettlesome, full of life and mettle, brisk, vigorous, wanton.  Bywiog ac eidiog ydyw, K.D. G. and T.A. i farch. (Antique Linguae Britannicae Thesaurus, Richards, 1815)

... Geoffrey calls the earl of Gloucester "Eldol," and not "Eidiol." (The Reader's Handbook of Famous Names in Fiction, Allusions, &c., Brewer, 1899)

    EIDIOL, or Eidol, is said in the Welsh Bruts to have succeeded Arthmael on the throne of Britain, in the second century B.C. (Myv. Arch. ii. 165.)
    EIDIOL (GADARN,) earl of Gloucester, is said, in the Welsh Chronicles, to have been the only Briton who escaped the memorable massacre of four hundred and sixty chieftains, who had been invited to an unarmed conference by the Saxons under Hengist, on Salisbury Plain, on which occasion having found a pole on the ground, he slew seventy of the enemy and escaped.  In a battle soon after, Eidiol had the satisfaction of taking Hengist prisoner, and afterwards beheading him.  Eidiol is also recorded in the Triads with Gwgon and Gwrnerth, as the three "gyrddion," or strong men of the Isle of Britain, and he is there said to have killed six hundred and sixty of the Saxons in the course of the day on which the massacre occurred.  (Myv. Arch. ii. 68, 255, 273.)  Nennius mentions the massacre, but does not name Eidiol.  He is, however, recognized by the English antiquaries, who call him Eldol or Edol, and say that he was earl of Gloucester, in the year 461.  (See Dugdale's Baronage; and Gibson's Camden.) (Enwogion Cymru, Williams, 1852).