Abbreviation for 'Pork Chop Sandwiches', a reference to the comic re-dubbing of a Public Service Announcement, shown as part of the American GI Joe cartoon. The re-dubbs were created by Eric Fensler, and have been popular on the web for many years.
The meaning of the term PCS relates to the events in the re-dubbed PSA, where a character called Blowtorch saves some retarded children from a fire they have caused, due to their ineptitude in the kitchen. His opening line is a shout of "Pork chop sandwiches!", and this is informally used as the film's title, which is actually "PSA07".
The PSA's denouement has the older of the two retarded children babbling incoherently to Blowtorch, resulting in his silent face, staring, motionless, as if 'in to camera', conveying the sentiment that there is really no possible response to such utterly nonsensical rubbish.
As such, it is used in text messages, web sites etc. as an exclamation in response to a situation where the absurdity, stupidity, perversity, or even inherent unfairness, is apparent to anyone intelligent, and as such needs no further explanation. To acknowledge a situation with 'PCS' is to convey that one understands exactly what the other person is feeling.
Kev: My manager just told me that he couldn't give me a raise because I'd not written any new articles for the intranet.
Steve: Didn't he tell you not to spend time writing those, as fixing customer crit. sit.s was more important?