Shimapan is an abbreviation of shima-pantsu (striped panties), are one of the new wave moe fashions to sweep the otaku landscape.

Briefly, "the realization that each random passerby is living a life as vivid and complex as your own." Originally from the Dictionary of Obscure Sorrows, which has a lot more beautiful neologism definitions like this you might enjoy.

Striped panties that are most recognized in the colours blue and white
Mostly featured in Anime.