late 14c., from L. secundum naturam "according to nature" (Augustine, Macrobius, etc.), lit. "following nature;" from medieval Aristotelian philosophy, contrasted to phenomena that were super naturam ("above nature," such as God's grace), extra naturam ("outside nature"), supra naturam ("beyond nature," such as miracles), contra naturam "against nature," etc.
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.