noun
Refers to a friend one has known since early childhood; often carries a sense of long-standing closeness.
彼は私の幼友達で、幼稚園からずっと一緒だ。
He is my childhood friend; we've been together since kindergarten.
Also means 'childhood friend', but 幼なじみ is more common in casual conversation, while 幼友達 is slightly more formal or literary.
Compound of 幼 (おさな, 'childhood') and 友達 (ともだち, 'friend'). The exact historical derivation is straightforward.