This is the 2231st most frequent Japanese word.
背
"背" means "back" or "spine" in English.
Here, 背 ('se') refers to the height or physical stature of an individual.
彼の背はとても高いです。
His stature is very tall.
In this sentence, 背 ('se') is used metaphorically to mean leaving behind.
彼女は誰も背にして前へ進みます。
She advances forward, leaving everyone behind.
Here, 背中 ('senaka') refers specifically to one's back and its posture.