Face that launched a thousand ships

Last Wednesday, I went out to tea with several friends, and one of the men said that he had a new girlfriend. I asked him what she looked like, and all he said was,"She has a face that launched a thousand ships."

What does this phrase mean? And what is its origin?

It is a reference to Helen of Troy. She was said to be so beautiful that, when she was abducted, a fleet of a thousand ships set sail to win her back from Paris, sparking the Trojan Wars.

If his girlfriend is even ¹/₁₀₀₀th the beauty of Helen of Troy, you can use the unit of facial beauty called the millihelen:

  –noun

1. (informal) A unit of measure of pulchritude, corresponding to the amount of beauty required to launch one ship.