pbbssh

"pbbssh", also spelled pbssh, is a sound made by dragging out the the "p" and "b" consonants through pressed lips, follow by a "sh" sound.

It is typically used to express disbelief or disagreement, sometimes with ridicule or disdain.

Disbelief:
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"I caught a 30 lb bass this weekend."
"Pbbssh, right! Pics or it didn't happen."

Disdain:
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"You gonna give up dem digits, baby?"
"Pbbssh, in yor dreams, playa!"