亲密无间

qīn mì wú jiàn

very close; inseparable

Character by character

亲qīn
close

密mì
intimate

无wú
no

间jiàn
gap

她们俩从小就亲密无间，什么话都说。

Those two have been inseparable since childhood and tell each other everything.

Tone: positive (praise).

间 is jiàn (a gap). For friends, couples and close partners.

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