my foot!
used to show that you do not believe something that someone has just said

[TahlilGaran] Dictionary of Contemporary English ▲

my foot
not possibly.
They call it the Thrifty Supermarket. Thrifty my foot — every time I go in there, it costs me a fortune.

Usage notes: used after a word or phrase to show you disagree with its meaning

[TahlilGaran] English Idioms Dictionary ▲

My foot!
[old-fashioned]
something that you say after repeating something someone has just said, in order to show that you do not believe it.
A fluent French speaker my foot! He knows a few words at the most.

[TahlilGaran] English Idioms Dictionary ▲

My foot!
I do not believe it (said after someone says something that you do not believe and you repeat the statement with the expression "My foot!")
"My foot!" the man exclaimed when he learned that he would not be able to attend the meeting.

[TahlilGaran] English Idioms Dictionary ▲