“Ful” is the Anglo-Saxon fúl (foulness), not ful (full); “some” is the affix meaning united with, the basis of something; as, gladsome, mettlesome, gamesome, lightsome, frolicsome, etc., etc.
“No adulation was too fulsome for her [Elizabeth], no flattery of her beauty too great.” —Green: Short History of England, chap. viii. sec. 3, p. 376.
Source: Dictionary of Phrase and Fable, E. Cobham Brewer, 1894