'aw' is an informal exclamation related to the word 'awe', which originates from Old English, specifically the word 'ege', where the root meant 'fear' or 'terror'.

'aw' developed from Middle English 'awe' (and earlier Old English 'ege') and evolved into the modern exclamatory form 'aw', used to express sympathy, pity, or gentle affection.

Initially the ancestor word meant 'fear' or 'terror'; over time the sense shifted through 'reverential fear' and 'wonder' to the modern exclamatory senses of sympathy, pity, or endearment.