"to toss" = "to throw carelessly, without effort; to discard" The phrase "like a stone" here is a simile (comparison) to something common or of little value. So, "to toss someone like a stone" is to discard a relationship suddenly, and seemingly without any thought or care, as if the relationship meant nothing special. You will hear / see other similar idioms: "to drop someone" (to end any relationship: boss/coworker, friend, lover, school/student, etc.) Ex.: "The college dropped me because of my grades." "to dump someone" (to end a dating relationship suddenly) Ex.: "Jenny dumped Craig for cheating on her." "to throw someone under the bus" (to cause others to blame someone, usually in order to avoid blame one's self or to achieve some gaine for one's self) Ex.: "George told the boss about the day I left early last week! I can't believe he threw me under the bus like that!"

Hi, another one of expressing this is 'to throw away like a match or piece of paper'.

Sorry for flood... Eric, your answers are always so complete, I learned a lot of new things...