This is from Old French sur, from Latin securus ‘free from care’. The same Latin word gives us security (Late Middle English), the legal surety (Middle English), and insecure (mid 17th century) ‘not free from care’, as well as assure (Late Middle English).
secure和sure同源，都来源于同一个拉丁语securus，不过传入的路径不同：secure是由拉丁语直接进入，sure是通过古法语进入的。se-前缀“without”，其实就是否定前缀；cure=care“担心，关心”；因为安全所以不用担心，因为确定所以不去在意。