dry behind the ears
see under wet behind the ears.
The Mystery Behind April’s NameWhile the first day of the fourth month of the year is sure to bring plenty of shenanigans, what isn't so certain is where the month's name originated.
The American Heritage® Idioms Dictionary Copyright © 2002, 2001, 1995 by Houghton Mifflin Harcourt Publishing Company. Published by Houghton Mifflin Harcourt Publishing Company.