when the upper front of a woman is shaken vigerously in a male or females direction to catch their attention. usually done to a very fast beat. sometimes (if the females are competitive) they will have a milkshake competition to see who can milkshake the fastest to get the most attetion.
man i was in a trace when my girlfriend milkshaked in front of me. then a saw her later, milkshaking to my best friend so i went to her best friend and she also gave me a milkshake which was way better than my girlfriend.