Source : Webster's Revised Unabridged Dictionary (1913)

Back \Back\, n. [As b[ae]c, bac; akin to Icel., Sw., & LG. bak,
   Dan. bag; cf. OHG. bahho ham, Skr. bhaj to turn, OSlav. b?g?
   flight. Cf. {Bacon}.]
   1. In human beings, the hinder part of the body, extending
      from the neck to the end of the spine; in other animals,
      that part of the body which corresponds most nearly to
      such part of a human being; as, the back of a horse, fish,
      or lobster.

   2. An extended upper part, as of a mountain or ridge.

            [The mountains] their broad bare backs upheave Into
            the clouds.                           --Milton.

   3. The outward or upper part of a thing, as opposed to the
      inner or lower part; as, the back of the hand, the back of
      the foot, the back of a hand rail.

            Methought Love pitying me, when he saw this, Gave me
            your hands, the backs and palms to kiss. --Donne.

   4. The part opposed to the front; the hinder or rear part of
      a thing; as, the back of a book; the back of an army; the
      back of a chimney.

   5. The part opposite to, or most remote from, that which
      fronts the speaker or actor; or the part out of sight, or
      not generally seen; as, the back of an island, of a hill,
      or of a village.

   6. The part of a cutting tool on the opposite side from its
      edge; as, the back of a knife, or of a saw.

   7. A support or resource in reserve.

            This project Should have a back or second, that
            might hold, If this should blast in proof. --Shak.

   8. (Naut.) The keel and keelson of a ship.

   9. (Mining) The upper part of a lode, or the roof of a
      horizontal underground passage.

   10. A garment for the back; hence, clothing.

             A bak to walken inne by daylight.    --Chaucer.

   {Behind one's back}, when one is absent; without one's
      knowledge; as, to ridicule a person behind his back.

   {Full back}, {Half back}, {Quarter back} (Football), players
      stationed behind those in the front line.

   {To be or lie on one's back}, to be helpless.

   {To put}, {or get}, {one's back up}, to assume an attitude of
      obstinate resistance (from the action of a cat when
      attacked.). [Colloq.]

   {To see the back of}, to get rid of.

   {To turn the back}, to go away; to flee.

   {To turn the back on one}, to forsake or neglect him.