The story
Compound of bosom + friend. Compare typologically Russian напе́рсник (napérsnik), akin to Proto-Slavic *pьrsь, Russian закады́чный друг (zakadýčnyj drug), akin to кады́к (kadýk)).
Etymology from Wiktionary, CC BY-SA 4.0.
bosom friend
Today it means: A very close friend. Full definition →