Brewer's: Bosom Friend
(A ). A very dear friend. Nathan says, “It lay in his bosom, and was unto him as a daughter.” (2 Sam. xii. 3.) Bosom friend, amie du cur. St. John is represented in the New Testament as the “bosom friend” of Jesus.
Source: Dictionary of Phrase and Fable, E. Cobham Brewer, 1894