Q: Am I right in assuming that the expression “wet behind the ears” refers to a newborn baby still wet with amniotic fluid?
A: Yes, the expression is believed to be an allusion to a wet newborn, but it first appeared in English in a negative version, “not yet dry behind the ears.”
The ultimate source of the usage, according to the Oxford English Dictionary, is the German “(noch) nass hinter den Ohren,” which showed up in the 1640s, and means “(still) wet behind the ears.”
The OED, an etymological dictionary based on historical evidence, describes the usage as “apparently with allusion to the idea that the area behind the ears is the last part of a newborn’s body to become dry after birth.”
A German version using “dry” showed up in the early 1700s as “(noch nicht) trocken hinter den Ohren,” meaning “(not yet) dry behind the ears,” and that’s the version that appeared in English in the early 1800s. The earliest English example in the OED is a translation of the negative German version:
“The French call such inexperienced uneducated boys, green creoles, (des créoles verts), as in German we usually say of such a person, ‘he is not yet dry behind the ears.’ ” (From the Aug. 21, 1802, issue of The Port Folio, a Philadelphia political and literary newspaper.)
The “wet” version of the expression appeared in English in the mid-19th century. The earliest Oxford example is from The Boston Daily Atlas, March 25, 1851:
“Such a louse student, who is still wet behind his ears, thinks because he is received in the castle, he is some great person!”