1 : toward the back : to or toward what is behind
a sudden movement backward, not forward
I heard a noise behind me and glanced backward.
He pushed the throttle backward.
2 : opposite to the usual way : in reverse
Count backward from 10.
3 : toward the past
a journey backward in time
The narrative moves backward and forward in time.
4 : to or toward a less advanced state or condition : toward a worse state
His new job, which paid less and had fewer responsibilities, felt like a step backward.
bend over backward or lean over backward also fall over backward
: to make a great effort to help someone or to reach agreement with someone
The salesman said he would bend over backward to make the deal.
I've leaned over backward to please you—and you haven't even said thank you!
know something backward and forward
— see KNOW