back 1 
(băk)
(băk)n.
1.
a. The part of the trunk of the human body along and to the sides of the spine between the neck and the pelvis; the dorsum.
b. The analogous dorsal region in other animals.
2. The backbone or spine.
3. The part or area farthest from the front.
4. The part opposite to or behind that adapted for view or use: the back of the hand; wrote on the back of the photograph.
5. The reverse side, as of a coin.
6. A part that supports or strengthens from the rear: the back of a couch.
7.
a. The part of a book where the pages are stitched or glued together into the binding.
b. The binding itself.
8. Sports
a. A player who takes a position behind the front line of other players in certain games, such as football and soccer.
b. In swimming, backstroke.
v. backed, back·ing, backs
v.tr.
1. To cause to move backward or in a reverse direction: Back the car up and then make the turn.
2. To furnish or strengthen with a back or backing.
3.
a. To provide with financial or material support: Unions backed the pro-labor candidate.
b. To lend moral support to, as by corroborating a claim. Often used with up: I'm not comfortable filing a complaint if you won't back me up.
c. To be in favor of; endorse or advocate: backed the reform proposal. See Synonyms at support.
4. To provide with musical accompaniment. Often used with up.
5. To bet or wager on.
6. To adduce evidence in support of; substantiate: backed the argument with facts.
7. To form the back or background of: Snowcapped mountains back the village.
v.intr.
1. To move backward: backed out of the garage.
2. To shift to a counterclockwise direction. Used of the wind.
adj.
1. Located or placed in the rear: Deliveries should be made at the back entrance.
2. Distant from a center of activity; remote.
3. Of a past date; not current: a back issue of a periodical.
4. Being owed or due from an earlier time; in arrears: back pay.
5. Being in a backward direction: a back step.
6. Linguistics Pronounced with the back of the tongue, as oo in cool. Used of vowels.
adv.
Phrasal Verbs: 1. At, to, or toward the rear or back.
2. In, to, or toward a former location: went back for the class reunion.
3. In, to, or toward a former condition: When the spell broke, the prince turned back into a frog.
4. In, to, or toward a past time: This story goes back to the 1920s.
5. In reserve or concealment: We kept back some money for emergencies.
6. In check or under restraint: Barriers held the crowd back.
7. In reply or return: emailed back that he would be late.
back away
To withdraw from a position; retreat.
back down
To withdraw from a position, opinion, or commitment.
back off
To retreat or draw away.
back out
1. To withdraw from something before completion.
2. To fail to keep a commitment or promise.
back up
Idioms: 1. To cause to accumulate or undergo accumulation: The accident backed the traffic up for blocks. Traffic backed up in the tunnel.
2. Computers To make a backup of (a program or file).
back and fill
1. Nautical To maneuver a vessel in a narrow channel by adjusting the sails so as to let the wind in and out of them in alteration.
2. To vacillate in one's actions or decisions.
back to back
Consecutively and without interruption: presented three speeches back to back.
behind (one's) back
In one's absence or without one's knowledge.
have got (someone's) back
To be prepared or ready to support or vouch for someone, as in a crisis.
have (one's) back up
To be angry or irritated.
off (someone's) back
No longer nagging or urging someone to do something.
on (someone's) back
Persistently nagging or urging someone to do something.
[Middle English bak, from Old English bæc.]
backless adj.