Phew! Etymology

I'm interested in the etymology of the expression "phew".

My online chambers dictionary says it's "a half-formed whistle". This leaves more questions than it answers, such as "why a half-formed whistle?" (as if I have a feeling of 'disgust, exhaustion, surprise, impatience, relief' I'm not tempted to whistle or half whistle) and "why is it spelt with a 'ph' rather than the more obvious 'f'?".

Various sources say the word originates from c.1600. I perhaps naively thought that words with 'ph' in them were in general imports from other languages (e.g. greek) where a 'phi' like letter existed.

So where does 'phew' come from?

Solution 1:

I would surmise that it came from an exaggerated sigh. When one is sighing out of boredom, frustration, and the like, one might take a deep breath and release the air through the side of one’s mouth in a puff-making a noise that, to my ear, sounds like a pew or phew. I’m guessing that phew is the official word for that utterance.