“a hundred days of sermonizing, one fart” → a metaphor for how something that one has put great effort into can come to nothing after even a slight mistake, much as a hundred days of serious spiritual study can be lost to the winds

(relativity) the theory of relativity

(rare) duality

influenza

corrosion resistance

(medicine) venereal disease (VD), sexually transmitted disease (STD)

(algebra) homogeneity

(psychiatry) bipolar disorder; manic depression

forward compatibility

backward compatibility