twinkle in one's father's eye

noun1

1. •
   (informal, humorous) A notional look of anticipation or hope (either for sex or a child) in one's father's eyes at or around the time of one's conception.

   • Kimberly handed him a whisky and said: "So's that, my boy. Forty-eight-year-old Glenlivet. This was a connoisseur's drop before you were a twinkle in your father's eye." — 1975