headlight

Etymology

head + light

noun

1. A bright light, with a lens and reflector, on the front of a motor vehicle (or originally a ship or train), designed to illuminate the road when driving at night; normally one of a pair.
2. (US, slang, vulgar, chiefly in the plural) A woman's breast.
   Oh, well. She's got... great... casabas.[…]Well, you know. Melons.[…]Hoo-has? Honkers? Hooters? Headlights? Uh... tatas? Teeters? Tweeters? Tom-toms? Tetons?[…]I'm trying to say it. Uh, meatballs. Mangoes. Cream pies. Cupcakes? Uh... bangers? Bouncers? Bulumbas?[…]Bazongas? Breasts! I said it. 1993, Quantum Leap, season 5, episode 14, spoken by Al Calavicci (Dean Stockwell)
3. (Canada, slang, vulgar, chiefly in the plural) A woman's erect nipples, partially masked by clothing.
4. (US, slang, chiefly in the plural) A jewel; especially a diamond.
5. (US, slang, chiefly in the plural) Lysergic acid diethylamide (LSD).