The Concise Oxford Dictionary defines a jack boot as a 'large boot worn above the knee, worn especially by 17th century to 18th century cavalry and by Nazi soldiers'. However, it also states the same definition is used figuratively to describe 'oppressive behaviour'.
If I had viewed the film scene which you describe, I would have assumed that the Irishman was making a figurative reference to his American landlady, comparing her attitude to that of a jack-booted Nazi.
Chris