Bottoms up!
Definition 1 of 1
Pro Tip 1/3Group Toast
Used during social gatherings when everyone is drinking together to signal a collective drink. 
At the wedding, the best man exclaimed 'Bottoms up!' and everyone took a sip.
Pro Tip 2/3Complete Drink
Implies finishing your drink in one go, not just taking a sip. 
His friend shouted 'Bottoms up!' and he chugged his beer.
Pro Tip 3/3Casual Occasions
Commonly used in casual, informal settings like parties, not formal events. 
At the barbecue, someone yelled 'Bottoms up!' as the music blared.