It ain’t a fit night out for man or beast.

In W. C. Fields’s comic persona—curmudgeonly, put-upon, and theatrically aggrieved—the line functions as a deadpan complaint about miserable weather and, by extension, miserable circumstances. The humor comes from its mock-solemn, folksy absolutism: conditions are so bad they exceed what even animals should endure, yet the speaker is (implicitly) out in it anyway. The phrase also evokes a vaudeville/early-film idiom of grumbling resilience, where exaggeration and rhythm (“man or beast”) sharpen the punch. As with many Fieldsisms, the line is less a literal observation than a compact performance of misanthropic wit and self-pitying bravado.