The need to
excuse oneself from festivities and goings-on and
retreat to the
bathroom, typically to make use of the facilities. Often done alone, although
women have been known to take bathroom breaks in
packs.
Usually the breakee taps the person seated next to him/her and says "I'll be right back, I need a bathroom break," and then stands and quietly departs. If anyone asks, "Hey, where'd (name) go?" the person whom the breakee spoke to will say "Oh, s/he needed a bathroom break."
Truly one of the finer points of casual ettiquite. This method is much more appropriate and much less crude than standing up from the group and announcing your intention to drain the lizard or some other colorful euphemism. Discretion is the better part of valor, after all.