The bond between childhood friends is the most genuine.
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From Li Bai's poem 《长干行》: '郎骑竹马来，绕床弄青梅' — 'You came riding a bamboo horse, circling the bench, playing with green plums.' The bamboo horse (a stick between the legs) is the original kid's hobbyhorse.
usage
Related but stronger: 青梅竹马 (qīngméizhúmǎ) — childhood sweethearts, with romantic implication. 竹马之交 stays platonic.
