back¹ (bak) n.

1 the part of the body opposite to the front; in humans and many other animals, the part to the rear or top reaching from the nape of the neck to the end of the spine
2 the backbone or spine
3 the part of a chair that supports one's back
4 the part of a garment or harness that fits on the back of a person or animal
5 physical strength [put some back into the work]
6 the rear or hind part of anything; part behind or opposite the front [the back of the room, the back of his leg]
7 the part or side of anything that is less often used, seen, etc. [the back of the hand; the back of a carpet, textile, etc.; the back of a knife]
2
a) the part of a book where the sections are sewed or glued together; part covered by the spine
b) the spine of a book
9 Mining the roof or overhead part of an underground passage
10 Sports a player positioned behind many of his or her teammates, as a running back in football or a halfback in soccer
–adj.
1 at the rear or back; behind
2 distant or remote [back country]
3 of or for a time in the past [a back copy of a newspaper, back pay]
4 in a backward direction; returning; reversed [a back step]
5 Phonet. articulated with the tongue in a position toward the back of the mouth said of certain vowels, as () in cool
–adv.
1 at, to, or toward the rear; backward
2 to or toward a former position or location
3 into or toward a previous condition
4
a) to or toward an earlier time
b) to or toward a later time
5 so as to keep in reserve or concealment [to hold back information]
6
a) in return or requital
b) in opposition or resistance
–vt.
1 to cause to move backward, or to the rear
2 to be at the back of; stand behind
3 to support or help, as with money, endorsement, etc.
4 to make a wager in support of; bet on
5 to get on the back of; mount
6 to provide with a back or backing
7 to form the back of
8 to sign on the back; endorse
9 to provide security for (a currency, loan, etc.)
–vi.
1
a) to move or go backward
b) to move (into a desired position) through the faulty performance of an opponent
2 to have the back in a certain place or direction [the house backs on a lake]
3 Meteorol. to shift counterclockwise (in the Northern Hemisphere) said of the changing direction of a wind: opposed to VEER1 (vi
3)
–SYN support
–back and fill
1 to handle sails so that they alternately spill wind and fill with wind, as in maneuvering in a narrow channel
2 to zigzag
☆3 to vacillate, as in a decision
–back and forth
1 to and fro
2 from side to side
–back away
1 to move or go backward
2 to withdraw from a position, stop holding to an attitude, etc.
–☆back down
–☆(in) back of at or to the rear of; behind
–back off
1 to move back a short distance
2 Informal back down
2 Informal to refrain or cease from pursuing or annoying; lay off
4 [Informal] to stop supporting a position, holding to an attitude, etc.
–back out
1 to withdraw from an enterprise
2 to refuse to keep a promise
–back out of
1 to withdraw from (an enterprise)
2 to refuse to keep (a promise)
–back up
1 to support or help
2 to move or go backward
3 to accumulate as the result of a stoppage
4 Baseball to take a position behind (a teammate) in case there should be a mishandled or erratically thrown ball
5 Comput. to make a standby or alternate copy of (data, a file, etc.)
–back water
1 to use oars, a propeller, etc. to move backward or prevent drifting
☆2 to withdraw from a position or a claim
–behind someone's backwithout someone’s knowledge or consent
–be (flat) on one's back to be ill, bedridden, or helpless
–☆get off someone's back [Slang] to stop nagging or harassing someone
–get one's back up [Informal] to become angry
–☆go back on [Informal]
1 to be faithless or disloyal to; betray
2 to fail to keep (a promise, one's word, etc.)
–have someone's back k [Informal] to watch over, defend, or support someone who is in a vulnerable position
–put someone's back up [Informal] to make someone angry
–turn one's back on
1 to show anger, contempt, etc. toward by turning away from
2 to ignore the plight of; desert; fail
–watch one's back [Informal] to be wary against an attack, specif. a treacherous attack
–with one's back to the wall in a desperate or awkward position or situation, often one in which one finally has to act in response to an attack or demand