A school one. 'Chin'.
Chin (schoolyard slang) - A word of great great derogatory power only to be uttered (and then loudly while laughing) upon the occasion of somebody saying something clearly untrue, such as 'I have a girlfriend, but she lives in Kent.'
'My completly real girlfriend is called Teresa Green'
or the classic, 'It wasn't me!'
Synonym: 'Reck-ON!' in an exaggerratedly disbelieving voice.
For added power it was accompanied by stroking an imaginary chin beard and looking thoughtful. To be cooly nasty, nodding without words while stroking the chin. Unless it's really bad, in which case it's 'chinny reckon!'.