second nature

second nature (uncountable)

1. (idiomatic) A mindset, skill, or type of behavior so ingrained through habit or practice that it seems natural, automatic, or without a basis in conscious thought.
   • 1825, Sir Walter Scott, The Talisman, ch. 1:

     But habit had made the endurance of this load of panoply a second nature, both to the knight and his gallant charger.

   • 1915, P. G. Wodehouse, Something New, ch. 1:

     Ashe Marson had realized that he must forego those morning exercises which had become a second nature to him.

   • 2005, Jeremy Caplan, "Game On, Hold The Pepperoni," Time, 3 April:

     For many video gamers, juggling a joystick and a pizza is second nature.

• Traditionally preceded by the indefinite article a, but now usually used without an article.