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.
See also: 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.
See also: able, not, something, toffee