侃大山 (kǎn dàshān) is a common colloquial phrase meaning 'to chat idly' or 'to shoot the breeze'. 侃 alone can also mean to boast or talk smoothly. It is informal and colloquial in register.
Left person radical, the side form of 人. Talking is a human activity, so the radical sets the scene: a person doing something with their mouth. Indexes 侃 in the chatter-and-action family alongside 你 you, 们 plural, 信 trust, 仁 benevolence.
Upper-right 口 - a mouth, the physical organ of speech. Sits on top of the flowing river 川 below, picturing words pouring out as freely as water from the mouth. This stacked image is the etymological core of 侃: easy, unhurried, free-flowing talk.
Lower-right 川 - three vertical strokes picturing a river. Below the mouth above it, this converts speech into water imagery: chat flowing on and on like a stream. Hence 侃 means to talk at length, easily, even to boast - 侃大山 means to chew the fat for hours.
