Lyric discussion by ohfaith 

Cover art for Suimasen lyrics by Blonde Redhead

actually "sumimasen" means "excuse me" and a whole bunch of other things in japanese. so I'm not sure if this misspelling is on purpose or not. :)

"Suimasen" is just a colloquial pronunciation of "sumimasen." Sometimes the Japanese write it like that on purpose the same way write "gonna" or "prolly."

In some cases it can have a kind of teasing tone, like if you made an inconsequential faux pas that people are more likely to laugh at than be mad at. "Oooops! Suimasen..."