Why did Egyptians swear on onions?

Answer 1

For the Egyptians, the onion was sacred. The nine encircling layers of an onion represented eternity. Onions were often used in Egyptians burials believing that the strong scent of the onion would bring breath back to the dead.

Answer 2

In ancient Egypt Onions were used for medicine... :)

They also used them in cooking. For poor people and slaves onions were an important part of their diet.