I’m going to list indeclinable, straight iconic words, as opposed to the far larger set of inflected words with an onomatopoeic etymology (like zuzuni for bug or platsurizo for to splash).
- bam bang, dan ding dong, apsu < Turkish hapşu sound of sneezing, kix cough, xrats scratch, drin ring ring, ksu shoo, prits blowing raspberry, ɣlu glug glug, sut shoosh, psit psst, ftu spit
- ɣav bark bark, njau meow, tsiu cheep cheep, kokoko cluck cluck, kokoriko cock-a-doodle-doo
Modern Greek also has a quite elaborate baby talk register partly based on onomatopoea. (In fact, Tsakonian has its own distinct set of baby talk words!)
- mam eat, dziz fire/danger, pipi pee, kaka poo, nani sleep, tsa peek-a-boo, nini baby, ata < Italian strata walkies, tutu car