mid-15c., an expression of surprise, delight, disgust, or pain in nearly all Indo-European languages, but not found in Old English (where the equivalent expression was la!), so perhaps from Old French a "ah!, oh! woe!"
1. "Ah, Captain Fox," Martin McGuinness said affably. "Nice to see you again."
2. Ah yes, but think of all the family life they're missing.
3. I'm meeting Anna Langenbach. Ah, this seems to be the train now.