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1.
1. To steal a sip of someone’s beverage.

2. At it’s most spectacular it is done with a straw, while the person is still holding their beverage.
There are lots of examples. If you go to facebook dot com slash ninjadrinking you will see a whole page dedicated to ninja drinking

Daniel Day Lewis showcased his ninja drinking skills in There Will Be Blood. Google "I drink your milkshake" and you will see the video on youtube
by drink ninja October 10, 2011