An affectionate low level hook-up often made in commiseration, with no serious emotional involvement on the part of the smasher, but sometimes inadvertently on the part of the smashee. Essentially a modern update of the 80s/90s 'Sympathy Fuck'.
Sue pity-smashed Tom after his break up with Helen, and now he thinks it's serious and won't leave her alone..
When in the act of having anal sex with your partner, you say you have to pull out for a second (to put on another condom or some other reason) then your friend switches in and starts having anal sex with her while you sneek around outside and wave at her through a window. then when she turns to look who is having sex with her your friend punches her in the face and at the same time you punch through the window.