as close as sock and boot;as thick as thieves; be cheek by jowl with; be closely united; be cup and can; be on very intimate terms with each other; be thick with sb; be very chummy with; be near and dear
❍ 他俩自幼两小无猜，如今更是～。They were innocent playmates in their childhood and now are on even more intimate terms.
亲密无间qīn mì wú jiàn
间：缝隙。形容非常亲密，毫无隔阂。as thick as thieves, be on very intimate terms with each other, near and dear, be hand and glove