transitive verb, ichidan verb
Because the hiragana ends with an う sound, you know this word is a verb. The kanji itself means tighten so the verb vocab version means to tighten.
You go to tighten and to tie your captive up, and you realize you don't have anything to do it with. Then you see a sheep standing nearby. Okay. You take the sheep (し), pull its wool, and tie the sheep and your captive up together around a tree. Problem solved!
Did you tightly turn off the gas?