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Used repeatedly, it is a noise of dimisal and disgust, usually reserved for when people are being rude. Roughly equivalent to "shut up".

Care must be taken when using this form of phtt, as the common reply is to spit on the person saying it.

Can also be used in conjunction with basying someone.
"Mate, this chicken isn't cooked, AGAIN. You're cooking's crap."
"Phtt phtt phtt phtt phtt phtt"
by Macrophage June 11, 2006