expression, noun
childhood friend; old playmate
Idiomatic expression for a close friend from childhood, often with nostalgic or literary overtones.
彼とは竹馬の友で、幼稚園の頃からずっと一緒だ。
He and I are childhood friends; we've been together since kindergarten.
幼なじみ is the common, everyday word for a childhood friend. 竹馬の友 is a more literary, idiomatic expression with a nostalgic feel.
Literally 'friend of the bamboo horse', referring to the bamboo stilts children used to play with. The phrase evokes the image of friends who played together on bamboo stilts in childhood.