Aaaahh! is the second alphabetic entry in A Lexicon of Cadet Language, Royal Military College, Duntroon in the period 1983-1985 (by Bruce Moore). Aaaahh! is defined as follows: "a cry expressive of the potentially disastrous consequences of activities such as mountain-climbing and parachuting".
I think we can all relate to that.
The great thing about a word such as this is that it doesn't have to be learned. It is simply uttered spontaneously under the right conditions. (Unlike "holy cow", "help", "sh** a brick", etc which have to be learned.)
As a member of the Corps of Staff Cadets in an earlier cohort, I should observe that there seems to have been a shift in pronunciation since my era. My recollection is that we screamed at greater length, thus: Aaaaaaarrrgh! (Note also the palatal fricative towards the end of the utterance.)