Rest mass energy of electron For electron's energy why we some time use the word "REST MASS ENERGY" why we use this word as electron is always spinning?                                                                                                      
 A: Don't think of the electron as a particle; it just looks like a particle when you observe it. The rest of the time, it is described by a wave function: a probability of finding it in a particular state if observed. We say the electron is "at rest" if the expected position, in our frame of reference, is independent of time - in other words, its center of mass is not moving.
"Spin" is a property of an electron that can be observed when the electron interacts with a magnetic field: the spin can be aligned with the field, or counter-aligned, and this gives rise to a different energy state. But we don't consider the electron as "moving" because it has spin. 
A: Please excuse me for rephrasing the question:

Why use the phrase "REST MASS ENERGY" when the electron is always spinning?

Because the electron isn't actually spinning. It isn't some billiard-ball thing rotating like a planet. Instead it has a wave nature, wherein electron spin is intrinsic spin. It's what makes it what it is. For an analogy, think of a tornado. It has intrinsic spin. If it didn't, it wouldn't be a tornado. If you could somehow remove that rotation, it wouldn't be a tornado any more. It's similar for an electron. It's a spin ½ particle. It has intrinsic spin. If it didn't, it wouldn't be an electron. If you could somehow remove that spin, it wouldn't be an electron any more.  
The nature of it is not clear in the literature, but note that the Einstein-de Haas effect demonstrates  "that spin angular momentum is indeed of the same nature as the angular momentum of rotating bodies as conceived in classical mechanics". Also Google on electron standing wave and note that in atomic orbitals electrons "exist as standing waves". A standing wave looks motionless, but it isn't. You can see an example of such hidden motion in the Poynting vector for static fields. There's a circular flow of electromagnetic energy. And whilst this isn't actually at rest, it isn't going anywhere with respect to you, so it's effectively at rest. 
You can relate this to Einstein's E=mc² paper: "The mass of a body is a measure of its energy-content". When you trap a massless photon in a mirror box, it increases the mass of that system. It's still going round and round inside the box at c, but it's effectively at rest, and the box is harder to move because the photon is in there. Then when you open the box, it's a radiating body that loses mass. But until you do, that energy is effectively at rest, and we call it rest-mass energy.          
