Wiktionary
quack like a duck
vb. (context simile English) To appear to be exactly what one is.
Usage examples of "quack like a duck".
But as you crow like a rooster or quack like a duck or moo like a cow, scrutinize your brother or sister Discordian with alert interest - never cracking a smile - to see how he or she will respond.
Unless while you're under, I can convince the hypnotist to make you quack like a duck.
If you miss the ball, you have to get down on your knees and quack like a duck.