Wet behind the ears means naive, immature, inexperienced. The idiom wet behind the ears is a reference to a newborn baby, still wet with amniotic fluid. It is an American phrase, coined around 1902, though Edward Bulwer-Lytton used the phrase not yet dry behind the ears in the novel The Parisians in 1873. The character who uttered the phrase was American. When used as a modifier before a noun wet behind the ears is hyphenated, as in wet-behind-the-ears.
The idiom dry back of the ears was an idiom that was also popular at the beginning of the twentieth century. Dry back of the ears means the exact opposite of wet behind the ears, it describes someone mature and experienced. Interestingly, the idiom dry back of the ears has gone by the wayside, while wet behind the ears is still a fairly popular idiom.
In fairness, few would have predicted it would come like this – the kind of red card you’d expect from a 19-year-old still wet behind the ears. (The Liverpool Echo)
After all, the slight error for the goal can happen to anyone, never mind someone still wet behind the ears. (The Telegraph)
In recent days, they transformed their establishment candidate of choice — safe, wet-behind-the-ears Marco Rubio — into a third-rate Andrew Dice Clay who tossed his usual stump speech and took to dissing Trump about his “small hands” and spray tan, as he tried, pathetically, to trump Trump with his own lame version of Trumpisms. (The Boston Herald)
The wet-behind-the-ears Warriors came home from Iolani’s preseason tournament last weekend with a hankering for more conditioning, so Lyons decided he’d dive head first into the problem, leading drills from the far lane at Naeole Pool. (West Hawaii Today)