'To lie on one's back' definitions:

Definition of 'To lie on one's back'

From: GCIDE

Back \Back\ (b[a^]k), n. [AS. b[ae]c, bac; akin to Icel., Sw., & LG. bak, Dan. bag; cf. OHG. bahho ham, Skr. bhaj to turn, OSlav. b[=e]g[u^] 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. [1913 Webster]
2. An extended upper part, as of a mountain or ridge. [1913 Webster]
[The mountains] their broad bare backs upheave Into the clouds. --Milton. [1913 Webster]
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. [1913 Webster]
Methought Love pitying me, when he saw this, Gave me your hands, the backs and palms to kiss. --Donne. [1913 Webster]
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. [1913 Webster]
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. [1913 Webster]
6. The part of a cutting tool on the opposite side from its edge; as, the back of a knife, or of a saw. [1913 Webster]
7. A support or resource in reserve. [1913 Webster]
This project Should have a back or second, that might hold, If this should blast in proof. --Shak. [1913 Webster]
8. (Naut.) The keel and keelson of a ship. [1913 Webster]
9. (Mining) The upper part of a lode, or the roof of a horizontal underground passage. [1913 Webster]
10. A garment for the back; hence, clothing. [Obs.] [1913 Webster]
A bak to walken inne by daylight. --Chaucer. [1913 Webster]
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 on one's back or To lie on one's back, to be helpless.
To put one's back up or to 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. [1913 Webster]