1. Geoffrey may have been born unexpectedly at a place called 'Les Botereaux', in Normandy while his mother was travelling. 'Botereaux' is the plural of 'Boterel' and means 'The place of toads.' So his shield might show where he came from.
2. Perhaps he had narrow shoulders and wide hips - like a toad, or like a leather bottle (a bota) - and it was a nick-name. But, if you looked like that, would you carry a shield with three toads and be laughed at? Besides, Buttriss is another spelling of the name. It's more likely to come from 'Botereaux' than 'bota'.
3. Rumour had it that he or his father had signed a pact with the Devil for family advancement, and the Devil had insisted the family bear his own coat of arms - 'argent, three toads erect sable.' Duke Eudes had something on William the Conqueror and this may have been a tit-for-tat. Neither Eudes nor Geoffrey would support the Conquest, though others of the family did, and did incredibly well out of it - a successful pact? No one really believed the story because the family was never Christian, and there is no Devil in the Old Religion.