sumaki 簀巻き

In Japanese, sumaki 簀巻き is a method of execution in which someone is rolled up in a bamboo mat and thrown in the river to drown. It may also refer to anime girls being wrapped in blankets like a burrito.

Character: Touwa Erio 藤和エリオ
Anime: Denpa Onna to Seishun Otoko 電波女と青春男 (Episode 1)

Meaning

The term sumaki 簀巻き means literally a "roll," maki 巻き of su 簀, which was a "mat" made out of pieces of split bamboo or reeds tied together.

It originates in an execution method in which someone was rolled up in such bamboo mat and then thrown in the river. Since they were bound, they wouldn't be able to swim, and would drown.

Example of sumaki 簀巻き, wrapping people with a mat and throwing them into the river.

Anime: Ojamajo Doremi Dokkaan! おジャ魔女どれみドッカ〜ン！ (Episode 17)

Nowadays, people aren't executed like this as much, so the term ends up referring rolling up someone like this as a prank, normally using a blanket and a string.