not be able to do something for toffee
can't (do something) for toffee
slang Is totally unable to do something. Primarily heard in UK. Oh, you'll definitely beat Sarah for that role in the musical—she's can't sing for toffee.
Farlex Dictionary of Idioms. © 2015 Farlex, Inc, all rights reserved.
not be able to do something for toffeebe totally incompetent at doing something. British informal
2000 Times Wordsworth himself couldn't spell for toffee, and his punctuation was extraordinarily bad.
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