1. The toss salad, or tossed salad, is a mixture of uncooked vegetables. Usually this means leafy ones like lettuce; spinach, chard, or dandelion leaves with just a smattering of other vegetables or foods as accents. However I have seen even a bean salad described as toss salad.
2. Origins of the slang meaning, to rim or lick another person's anus as part of a sexual act is uncertain. It has been noted as slang for quite some time among college students, at least since the late 1970's. It has also evolved into meaning to be angrily yelled at by someone in authority, similar to "having your ass chewed out" in some areas.
Some people guess that it may have something to do with the slang "toss", meaning rubbish or an act of masturbation. Similar are the words "tosser" and "toss-pot", both insulting names for an idiot or contemptible person.