What is "to knock under"

Knock \Knock\ (n[o^]k), v. i. [imp. & p. p. Knocked (n[o^]kt); p. pr. & vb. n. Knocking.] [OE. knoken, AS. cnocian, cnucian; prob. of imitative origin; cf. Sw. knacka. Cf. Knack.]

1. To drive or be driven against something; to strike against something; to clash; as, one heavy body knocks against another.
   --Bacon.

2. To strike or beat with something hard or heavy; to rap; as, to knock with a club; to knock on the door.

   For harbor at a thousand doors they knocked.
   --Dryden.

   Seek, and ye shall find; knock, and it shall be opened unto you.
   --Matt. vii. 7.

3. To practice evil speaking or fault-finding; to criticize habitually or captiously. [Slang, U. S.]

   To knock about, to go about, taking knocks or rough usage; to wander about; to saunter. [Colloq.] ``Knocking about town.''
   --W. Irving.

   To knock up, to fail of strength; to become wearied or worn out, as with labor; to give out. ``The horses were beginning to knock up under the fatigue of such severe service.''
   --De Quincey.

   To knock off, to cease, as from work; to desist.

   To knock under, to yield; to submit; to acknowledge one's self conquered; -- an expression probably borrowed from the practice of knocking under the table with the knuckles, when conquered. ``Colonel Esmond knocked under to his fate.''
   --Thackeray.