|a habit, characteristic, etc, not innate but so long practised or acquired as to seem so|
|a calculus or concretion found in the stomach or intestines of certain animals, esp. ruminants, formerly reputed to be an effective remedy for poison.|
|a children's mummer's parade, as on the Fourth of July, with prizes for the best costumes.|
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.