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
? 他倆自幼兩小無猜，如今更是～。They were innocent playmates in their childhood and now are on even more intimate terms.
親密無間qīn mì wú jiàn
間：縫隙。形容非常親密，毫無隔閡。as thick as thieves, be on very intimate terms with each other, near and dear, be hand and glove