This question used to come up a great deal on a newsgroup far away, so often in fact I took the time to type out a full explanation for the edification of newcomers who had since parachuted in and thought there must be something a tadge salacious about the term. Been a long time since I've had to hunt this one up from out of the files, but wot the heck:

City boys, I am surrounded by city boys....

"ORK" is the acronym for ortho-rhombic-kainite, a compound developed about five years ago to kill a particularly resistant bovine parasite. Though highly effective in 86'ing the little nasties, it's also incredibly gentle on the cow (unlike earlier preparations which when strong enough to be any good, often caused troublesome skin lesions). ORK's gentleness is in part due to its heavy dilution -- you mix one part ORK with 60 parts water. However, because the solution is so weak, one treatment isn't enough -- cattle have to be sprayed once a day for an entire season.

Most farmers mist each infested cow with the solution just before the evening milking. Orking an entire herd can take hours, which is why your average farmer will press a couple of his kids into the job. Now, there's nothing glamourous about spraying down a cow as it comes in from the field. It's a tiring, messy business. That's why kids who get bamboozled into doing this are awarded the title of "cow orker" -- at least it's a fancy name they can brag about when they go on and on about how they helped Daddy.

The term "cow orker" has hence come to mean anyone who does a particularly nasty and thankless job solely for the recognition. The term has moved off the farm, so to speak -- that's why you see references to "cow orkers" in all kinds of non-agricultural workplaces.

And no, there's nothing illegal about cow orking. Just ask anyone you work with.