A mocking response to a non sequitur, or a nonsensical explanation of something that doesn't need explaining. SOURCE: Bernstein, Jüdische Sprichwörter und Redensarten. Variation:
.דעריבער גייען קאַטשקעס באָרוועס און גענדז אָן פּלודערן
Deriber geyen katshkes borves un gendz on pludern.
Therefore ducks go barefoot and geese don't wear trousers.
SOURCE: Stutchkoff, Der Oytser fun der Yidisher Shprakh