'To knock under' definitions:

Definition of 'To knock under'

From: GCIDE

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. [1913 Webster]
2. To strike or beat with something hard or heavy; to rap; as, to knock with a club; to knock on the door. [1913 Webster]
For harbor at a thousand doors they knocked. --Dryden. [1913 Webster]
Seek, and ye shall find; knock, and it shall be opened unto you. --Matt. vii. 7. [1913 Webster]
3. To practice evil speaking or fault-finding; to criticize habitually or captiously. [Slang, U. S.] [Webster 1913 Suppl.]
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. [1913 Webster]