Wet behind the ears.
What’s being wet behind the ears got to do with being inexperienced? Well, let me break it down for you. According to this idiom’s origin, it is based on the idea that newborn animals are totally wet from the amniotic sac during birth. The mother would then proceed to lick the baby animal thoroughly, but the indentation behind the ears (usually with a baby calf) would still be wet.
Meaning: Used to describe someone inexperienced or immature.
Example: That lady thinks she’s the boss of me! She’s still wet behind the ears!