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1.
Some may say this is a cheap beer.. but there is only one real beer that may be called frat water.. and that would be Fratty Light, aka Natural Light.
fratdaddy: where's my koozie? i need it for this frat water.

gdi: what's frat water?
by The Vines December 10, 2010
 
2.
Cheap beer. Preferably Keystone, Natty, or the Beast. Acceptable substitutes include Coors or Bud Light.
"Put down your cranberry vodka and drink some frat water like a real man."

"Be a gentleman and give her some frat water before you take down"
by JFitz October 27, 2006
 
3.
n) A Natural Light, Natty, or other coloquialism referring to the fraternity world's take on bottled water. See also fratty light.
I need some that fratwater to kill the hangover
by chet masterson III February 09, 2007
 
4.
Beer, usually very cheap such as Nati, BEAST, or Stones. Used because it is easier to find a beer in a fraternity than a sink that works.
Man Josh was so wasted off of frat water it was frattastic.
by Andrew Corcoran June 12, 2008
 
5.
The water that is derived from day old ice which has pooled into the trough under the keg.

If left outside, this water must contain at least one cigarette butt, bug and/or noticeable dirtiness. If left inside there are no requirements, but the water is considered majestically fratty if there are cigarettes in it.
I decided i needed to turn up the frat-meter and take a shot of frat water with mikey jays and it got fratted out of my B hole, REAL QUICK LIKE!
by CLE SMAK EE July 28, 2009