(bak) noun
1. The part of the body nearest the spine; in man the hinder, in quadrupeds the upper part, extending from the neck to the base of the spine. ◆ Collateral adjective: dorsal, fergal.
2. The backbone.
3. The rear or posterior part: Sit in the back of the car.
4. The farther or other side; the part away from the beholder: the back of the door.
5. The part which comes behind or is opposite to the part used in the ordinary movements of a thing: the back of a knife, the back of the hand.
6. The part of the leaves of a book sewed together into the binding; also, the part of the binding around this part.
7. The lining attached to the unexposed side of a thing, as for reinforcement.
8. In football, a member of the offensive or defensive backfield.
9. The ridge of a hill.
10. Phonet. The part of the tongue directly behind the front and below the velum.
—at one's back Following closely.
—behind one's back
1. Secretly.
2. Treacherously.
—in back of Colloq. Behind; to the rear of.
—to be (flat) on one's back To be helplessly ill.
—to get (or put) one's back up To become (or make) angry or obstinate.
—to put one's back into To exert all the physical strength of which one is capable.
—to turn one's back on To show contempt or ill feeling toward by turning away from or ignoring.
—with one's back to the wall Cornered; having no issue save by fighting one's way out.
—v.t.
1. To cause to move or go backwards; force to the rear: often with up.
2. To form the back of; supply with a back.
3. To strengthen at the back.
4. To support, assist, or uphold; be in favor of: often with up.
5. To support financially.
6. To bet on the success or chances of.
7. To mount, sit, or ride on the back of, as a horse.
8. To write on the back of; address or endorse, as a check.
—v.i.
9. To move or go backward: often with up.
10. To shift counterclockwise: said of the wind: opposed to veer.
—to back and fill
1. Naut. To keep (a vessel) in mid-channel by alternately filling and spilling the sails, so as to be advanced by the current alone.
2. To be irresolute; vacillate.
—to back down To withdraw from a position, abandon a claim, etc.
—to back off To retreat, as from contact.
—to back out To withdraw from or refuse to carry out an engagement or contest.
—to back water
1. To retard the progress or reverse the motion of a vessel by reversing the action of the oars or of the propelling machinery.
2. To withdraw from a position; retract, as a claim.
—adjective
1. In the rear; behind: a back room.
2. Distant; remote: the back country.
3. Of or for a date earlier than the present: back taxes.
4. In arrears; overdue, as a debt.
5. In a backward direction: a back thrust.
6. Phonet. Describing those vowels produced with the tongue pulled back in the mouth, as () in food. [OE bœc back]

(bak) adverb
1. At, to, or toward the rear: to move back.
2. In, to, or toward a former place: to go back home.
3. In, to, or toward a former condition: My cold has come back.
4. In time past; ago: years back.
5. In return or retort: to talk back.
6. In reserve or concealment: to keep something back.
—back and forth First in one direction and then in the opposite.
—back of Behind.
—to go back on Colloq.
1. To refuse to keep a promise or engagement.
2. To desert or betray (a cause). [<ABACK]

(bak) noun
A brewer's tub or vat. [<Du. bak trough <F bac tub, basin]