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.
not be able to do something for toffee
be totally incompetent at doing something. British informal 2000 Times Wordsworth himself couldn't spell for toffee, and his punctuation was extraordinarily bad.