Rain Dogs And Cats Meaning

(idiomatic, dated) To rain very heavily.

Example: 1651 - Henry Vaughan, Olor Iscanus (1651).
  The Pedlars of our age have business yet,
  And gladly would against the Fayr-day fit
  Themselves with such a Roofe, that can secure
  Their Wares from Dogs and Cats rain'd in showre.
1672 - Maurice Atkins, Minor Burlesques and Travesties (1672)
  Where e're I went on Land or water
  Hee'd make a shift to follow after.
  Neither had he flincht a foot, had fates
  Made it rain down dogs and cats
1678 - John Phillips, Maronides (1678)
  When it rains Dogs and Cats in Hell,
  The shelter'd Centaurs roar and yell;