DEFINITION: a rat used by researchers conducting medical experiments. And by extension, any animal or human made the test subject of a laboratory experiment.
EXAMPLE: ' The White Smocks gave gave each of them a test tube and said they wanted a sperm count. What do you mean? Place your sperm in the tube. How? Through ejaculation. Just like that? Masturbation is the customary procedure. What! The best results seem to be obtained through fantasization, accompanied by masturbation, followed by ejaculation. Where, f'r chrissake? Use the bathroom. A couple of the boys said things such as, "Well, okay, I'll do it if you'll send a nurse in with me--to help me along if I get stuck." The White Smocks looked at them as if they were schoolboys making obscene noises. This got the pilots' back up, and a couple of them refused, flat out. But by and by they gave in, and so now you had the ennoblingprospect of half a dozen test pilots padding off one by one to the head in their skivvies to jack off for the Lovelace Clinic, Project Mercury, and America's battle for the heavens. Sperm counts were supposed to determine the density and motility of the sperm. What this had to do with a man's fitness to fly on top of a rocket or anywhere else was incomprehensible. Conrad began to get the feeling that it wasn't just him and his brother lab rats who didn't know what was going on. He now had the suspicion that the Reflector heads didn't know either. They had somehow gotten carte blanche to try out any goddamned thing they could think up--and that was what they were doing, whether there was any logic to it or not. '
--- 1979. TOM WOLFE. The Right Stuff. "Chapter 4 -- The Lab Rat." (Page 74 - 75 ). Bantam Book edition (ISBN 0-553-27556-9).