a (free) glass of water at a restaurant ordered to avoid an "excessive" bill and, in turn, extra tip. This is more pronounced when one member of the party orders a bottomless soft drink to share.
"on the rocks" - with ice
"d00d, wtf..." - hot water
Customer: We'll have two ice waters please.
**server 1 walks back**
Server 2: Can I help you out with anything?
Server 1: Yeah, two asian cocktails on the rocks...