be·hind 
(bĭ-hīnd)
(bĭ-hīnd)adv.
1. In, to, or toward the rear: We walked behind.
2. In a place or condition that has been passed or left: I left my gloves behind.
3. In arrears; late: I fell behind in my payments.
4. Below the standard level; in or into an inferior position: Don't fall behind in class.
5. Slow: My watch is running behind.
6. Archaic Yet to come or in reserve: "the expectation of some new possession, or of some enjoyment yet behind" (Samuel Johnson).
prep.
1. At the back of or in the rear of: He sat behind her.
2. On the farther side or other side of; beyond: The broom is behind the door.
3. In a place or time that has been passed or left by: Their worries are behind them.
4.
a. Later than: The plane was behind schedule.
b. Used to indicate deficiency in performance: behind us in technological development.
5.
a. Hidden or concealed by: hatred hidden behind a bland smile.
b. In the background of; underlying: Behind your every action is self-interest.
6. In a position or attitude of support: The leaders have the army behind them.
7. In pursuit of: The police were hard behind the escapees.
n.
Informal The buttocks.
[Middle English bihinde, from Old English behindan; see ko- in the Appendix of Indo-European roots.]