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Related to loosen tongue: loose tongue
loosen (one's) tongue
To cause one to speak uninhibitedly. Often refers to the effect of alcohol. A few drinks will loosen her tongue, and then I'm sure she'll tell me the secret without me even having to ask. I've never met a reporter like Courtney. She can loosen the tongue of even the cagiest politician.
loosen (someone's) tongue
To cause (someone) to speak freely or carelessly or to divulge information.