A' ae oo comes from an imaginary conversation in Scots about wool, first noted by Alan Ramsay in the glossary to his poems in 1721.

Customer ( Inquiring of the material ) : Oo?  ( Wool? )
Shopkeeper : Ay oo  ( Yes, it's made of wool )
Customer : A' oo?  ( All wool? )
Shopkeeper : Ay, a' oo  ( Yes, all wool )
Customer : A' ae oo?  ( All the same wool )
Shopkeeper : Ay a' ae oo  ( Yes all the same wool ).