That they would be prepared to have sex with that woman, at least once
1. It did not cost much in time or effort, and
2. If nobody else knew about it, and
3. If there were no strings attached - the man could walk away afterwards with no obligations or commitments.
So, for a woman to be described as 'sexy' isn't really a very high bar to clear, is it?
And it isn't really something a woman should regard as much of a compliment, is it?
And it isn't really something in search of which a woman should go to great lengths of artificially sculpting her body, make-up, diet, exercise, piercing and tattooing, wearing revealing and tight clothing, and - in general - presenting herself as the kind of person who seems likely to fulfil the above three criteria...