It's a Japanese word for "Death God" It is a demon that makes humans want to die, and decides their time of death.
Yes some one can be a shinagami i think that if you were to use a deathnote book and get the shinagami eyes and die after a while you can maybe become a shinagami but that comes from me so it may not be real because ofcourse i have not used the deathnote book
死神 /shi ni ga mi/ means 'death god' in Japanese.