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"

Support, especially financial.

"It's a volunteer organization that works with backing from the city and a few grants."

A liner or other material added behind or underneath.

"The cardboard backing gives the notebook a little extra stiffness."

A backdrop.

Musicians and vocalists who support the main performer.

The mounting of a horse or other animal.

The action of putting something back; a switching into reverse.

Providing support for the main performer.