没道理。我认为a fat chance字面上仍然表示「机会很大」，与a slim chance相对。只不过实际使用时总是以反讽的口吻来说的，所以理解为「没啥机会」。
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因为有人还是不信，我查了OED，很明确地说了 a fat lot 和 fat chance 都是反讽用法：
fat, a. and n.
A. adj.
A.III. With reference to the amount of produce or supply.
A.III.10. Well supplied with what is needful or desirable.
A.III.10.a. Of a person: Affluent, wealthy. Obs.
A.III.10.b. Of things: Abundant, plentiful; esp. of a feast, pasture, etc. Also, Well-stocked.
A.III.10.c. a fat lot: a large amount, a great deal: always ironical and implying ‘very little, hardly anything’. Similarly a fat chance, implying ‘hardly any opportunity (or possibility)’; also (N.Z.) a fat show.
例句显示这种反讽的用法在1892年就已经有了。
