The rear of the body, especially the part between the neck and the end of the spine and opposite the chest and belly.

"Could you please scratch my back?"

That which is farthest away from the front.

"He sat in the back of the room."

Upper part of a natural object which is considered to resemble an animal's back.

"The small boat raced over the backs of the waves."

A support or resource in reserve.

The keel and keelson of a ship.

"The ship's back broke in the pounding surf."

The roof of a horizontal underground passage.

Effort, usually physical.

"Put some back into it!"

A non-alcoholic drink (often water or a soft drink), to go with hard liquor or a cocktail.

"Could I get a martini with a water back?"

Among leather dealers, one of the thickest and stoutest tanned hides.

To go in the reverse direction.

"the train backed into the station;  the horse refuses to back"

To support.

"I back you all the way;  which horse are you backing in this race?"

(of the wind) To change direction contrary to the normal pattern; that is, to shift anticlockwise in the northern hemisphere, or clockwise in the southern hemisphere.

(of a square sail) To brace the yards so that the wind presses on the front of the sail, to slow the ship.

(of an anchor) To lay out a second, smaller anchor to provide additional holding power.

(of a hunting dog) To stand still behind another dog which has pointed.

To push or force backwards.

"The mugger backed her into a corner and demanded her wallet."

To get upon the back of; to mount.

To place or seat upon the back.

To make a back for; to furnish with a back.

"to back books"

To adjoin behind; to be at the back of.

To write upon the back of, possibly as an endorsement.

"to back a letter;  to back a note or legal document"

(of a justice of the peace) To sign or endorse (a warrant, issued in another county, to apprehend an offender).

To row backward with (oars).

"to back the oars"

A large shallow vat; a cistern, tub, or trough, used by brewers, distillers, dyers, picklers, gluemakers, and others, for mixing or cooling wort, holding water, hot glue, etc.

A ferryboat.