(idiom) to use a reputable name as a cover for dishonest deeds; to pass off low-quality goods or services as something better; dishonest advertising

Literally hanging a sheep's head while selling dog meat; describes dishonest advertising, deceptive practices, or doing wicked things under a virtuous name.

friends who are only around for food and drinks but cannot be relied upon in times of trouble

to display a sheep's head while boiling dog meat; to use a false facade to deceive people or sell inferior goods

To display a sign for high-quality goods while selling something inferior; figuratively, to do bad things under the guise of an honorable name.

to cheat or use dishonest advertising; literally "to hang a sheep's head while selling dog meat", describing wicked deeds carried out under a banner of virtue