a. 1. Next in order after nine hundred and ninty-nine; coming last of a thousand successive individuals or units; -- the ordinal of thousand; as, the thousandth part of a thing. [ 1913 Webster ]
2. Constituting, or being one of, a thousand equal parts into which anything is divided; the tenth of a hundredth. [ 1913 Webster ]
3. Occurring as being one of, or the last one of, a very great number; very small; minute; -- used hyperbolically; as, to do a thing for the thousandth time. [ 1913 Webster ]