What gets on your nelly?(both)

One thing that really flabbergasts me is Onomatopoeia words.
I mean to ask, how is one supposed to pronounce
Arghh?
Clunketh
Pshaw
Swaap
Thwapp
oooff
Made up words should never be allow
Who made up the word Onomatopoeia?
I bet it was those dastardly Greeks😡