It comes from Plutarch--I was only reading this last night...strange.
Below is a quote from source in question:
'It is said of Helvia, the mother of Cicero, that she was well born and lived an honourable life; but of his father nothing can be learned that does not go to an extreme. For some say that he was born and reared in a fuller's shop, while others trace the origin of his family to Tullus Attius, an illustrious king of the Volscians, who waged war upon the Romans with great ability. However, the first member of the family who was surnamed Cicero seems to have been worthy of note, and for that reason his posterity did not reject the surname, but were fond of it, although many made it a matter of raillery. For "cicer" is the Latin name for chick-pea, and this ancestor of Cicero, as it would seem, had a faint dent in the end of his nose like the cleft of a chick-pea, from which he acquired his surname. Cicero himself, however, whose Life I now write, when he first entered public life and stood for office and his friends thought he ought to drop or change the name, is said to have replied with spirit that he would strive to make the name of Cicero more illustrious than such names as Scaurus or Catulus. Moreover, when he was quaestor in Sicily and was dedicating to the gods a piece of silver plate, he had his first two names inscribed thereon, the Marcus and the Tullius, but instead of the third, by way of jest, he ordered the artificer to engrave a chick-pea in due sequence. This, then, is what is told about his name.' Plutarch, life of Cicero, I