Twinkle in my eye

Anyone know the origin of "before you were a twinkle in my eye?" And the actual definition of what a twinkle in the eye means, to clear up a little dispute. I can't find anything anywhere.

According to Cambridge:
when sb was a (mere) twinkle in their father's eye (HUMOROUS)- at a time before someone was born

According to askoxford:
a twinkle (or gleam) in someone's eye - something that is as yet no more than an idea or dream

Jenny, if you're 18 or over, we can explain to you why a birth might be associated with a previous twinkle in a man's eye. If you're 10 or over, we shouldn't have to.