inch tau mil
"Tau" or "Thou" in context you describe is slang expression for one thousandth and it refers to units in inches. "Mill" is same that comes from "milli" used in SI system and is also refering to 1/1000 of inch. "Thou" is more used in machining, while mill is more used in design of PCB's. In essence, just stupid sleng that improperly uses measurement units, because some people are to lazy to pronounce "milli inch" or "one thousandth of inch".
Another stupid one to watch for is "micron". In order to know which unit speaker refers to, you have to clarify if he is referring to "micro inch" or "micro meter". If you are trying to clarify dimensions on project for your company machinist, you might get empty stare as they might never new that "Micron" could mean something else other then micro-inch.