Literal Meaning

'it is extremely rare', when a person with ascites develops a hunched back

Implied Meaning

This occurrence is very rare and it will not likely happen again

Origin

Ascites  is a disease which causes fluids to accumulate in the abdomen, resulting in a swollen belly, which is projected forward while standing. Consequently, the afflicted person will acquire a straight back. It is inconcievable for the same person to develope a hump, or a bent back because the two - ascites and the hunched back - are incompatible