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1.
A lot.
To signify a lack of knowledge of some numerical data.
An answer to use when you want to annoy or simply not answer a question correctly.

Can also be spelt '12ty'
'I can well do more bounces on a pogo stick'
'No no, i can do twelfty more than you!'

'What time is it sir?'
'Twelfty'
by Luka Atkinson June 27, 2008