One's mouth, especially of a gossipy person.(noun)
One who yaps.(noun)
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Use "yapper" in a sentence
"Anytime this no good windbag opens her yapper we all should be very afraid."
"I did once have a neighbor who owned a mastiff and a Lhasa apso, they got along great and it was a scream to watch the little dog be alpha, the huge mastiff was completely dominated by the little yapper, but would defend it to the death."
"My specific role was to speak about birth, being that I'm a Birthing From Within childbirth mentor, but I have had experience read: drama in fertility and parenthood as well, so I was able to open up my yapper about all of it."