second nature

second nature idiom
A habit or mode of behavior so long practiced that it seems innate, as in Driving in heavy traffic is second nature to Chris. This expression is a shortened form of an ancient proverb, Custom (or usage) is a second nature, first recorded in 1390. It alludes to the fact that very frequently repeating something makes it seem completely natural or inborn.

The American Heritage® Dictionary of Idioms by Christine Ammer. Copyright © 2003, 1997 by The Christine Ammer 1992 Trust. Published by Houghton Mifflin Harcourt Publishing Company. All rights reserved.

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