E. Cobham Brewer 18101897. Dictionary of Phrase and Fable. 1898.
Bo or Boh,
in old Runic, was a fierce Gothic captain, son of Odin. His name was used by his soldiers when they would take the enemy by surprise. (Sir William Temple.)
From this name comes our bogie, a hobgoblin or little Bo. Gifford Castle is called Bo Hall, being said to have been constructed by bogies or magic. Compare Greek, boi, bah! verb, boaô, to shout out; Latin, böo, to bellow like a bull (bos). (See BOGIE.)
You cannot say Bo! to a goosei.e. you are a coward who dare not say bo! even to a fool. When Ben Jonson was introduced to a nobleman, the peer was so struck with his homely appearance that he exclaimed, What! are you Ben Jonson? Why, you look as if you could not say Bo! to a goose. Bo! exclaimed the witty dramatist, turning to the peer and making his bow. (Latin, bo-are; Greek, boa-ein, to cry aloud.)