the IPA(International Phonetic Alphabet) letter called the Voiceless Dental Liquid Velar Impossible Fricative or /e+?/ for short. It is quite literally impossible to pronounce and even linguists only theorize how it sounds.

Noun: an impossible sound i guess

Verb: to ɚ̴̥̤̪̬̰̺̼̻̹̜̟̠̝̩̞̯̘̃̃̃̊̆̈̽̋́̄̀̏̌̂̚ meaning to be in despair trying to pronounce it
Johnny: hey, wassup?

Bob: ɚ̴̥̤̪̬̰̺̼̻̹̜̟̠̝̩̞̯̘̃̃̃̊̆̈̽̋́̄̀̏̌̂̚

Johnny: whaaaaaaaaaaaaaaaaat????????????

*Bob magically disapears as the sound is impossible to make so he was imaginary the whole time*

Johnny: WAIT YOURE TELLING ME MY BEST FREIND BOB NEVER EXISTED????? WHAAAAAAAAAAA-
Get the ɚ̴̥̤̪̬̰̺̼̻̹̜̟̠̝̩̞̯̘̃̃̃̊̆̈̽̋́̄̀̏̌̂̚ mug.