dry behind the ears

Adjective

 * 1)  Seasoned or experienced; mature, especially with respect to judgment.
 * 2) * 1850, Springfield Rep., (Whig.), Daily (Columbus) Ohio Statesman, 9 May, p. 3, col. 4:
 * Why, you irreclaimable donkey, don’t you know the “notice” was an advertisement? When will you get dry behind the ears?

Usage notes

 * Mostly in the negative: "not dry behind the ears yet".