背縫い is read as せぬい (senui) and means back seam.
せぬい
senui
せがひくい
short (of a person)
せのひくい
せのたかい
tall (of a person)
せたけ
stature
せすじ
seam down the back (of clothing)
はいきんりょく
strength of one's back
はいはんこうい
act of disobedience (betrayal)
はいけいはんしゃりつ
background reflectance
はいしんこうい
breach of faith
はいけいほうしゃ
background radiation
はいけいまく
backdrop
はいすいのじん
fighting with one's back to the wall