Something (or someone) that you just can't get off your mind. Usually a song that plays over and over again in your head, but can also apply to that certain someone that you just can't stop thinking about. (;
A: Hey, that song on the radio was pretty nice.
B: Yeah haha, that song's been SIMH for almost a week now.
A: You've been staring at that girl over and over agian. What's up?
B: Dude, she's been SIMH the whole day! I think I like her... <3