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To make an mistake in anything to do with numbers, whether directly to do with an equation of some sort, or to do with days of the week etc.

Pronounced "M'thstake", as if trying to say Mistake with a lisp.

Can be used to annoy people when they deliberately exaggerate a number to make their statement carry more weight or if they are being patronirical.
Guy - I did like 400 push ups before
Girl - I think you made a slight mathstake, I counted about 15.
by Jack A....... November 09, 2008