Readily and without apparent effect. For example, The scathing reviews rolled off him like water off a duck's back. This expression alludes to the fact that duck feathers shed water. [Early 1800s]
The American Heritage® Idioms Dictionary Copyright © 2002, 2001, 1995 by Houghton Mifflin Harcourt Publishing Company. Published by Houghton Mifflin Harcourt Publishing Company.