Perhaps from the old practice of knocking under the table with the knuckles, when conquered. Compare knuckle under

knock under (third-person singular simple present knocks under, present participle knocking under, simple past and past participle knocked under)

1. (intransitive, dated) To acknowledge oneself conquered; to submit, to yield.
   • 1823, [James Fenimore Cooper], chapter XIV, in The Pioneers, or The Sources of the Susquehanna; […], volume II, New York, N.Y.: Charles Wiley;  […], →OCLC, page 209:

     I pull the second best trigger in this county. I knock under to Natty Bumppo there, sin' he shot the pigeon.

   • 1852, William Makepeace Thackeray, “I Come to an End of My Battles and Bruises”, in The History of Henry Esmond, Esq. […] , volume III, London: […] Smith, Elder, & Company, […], →OCLC, page 4:

     When he heard this news of Beatrix's engagement, Colonel Eſmond knocked under to his fate, and reſolved to ſurrender his ſword, that could win him nothing now he cared for; […]

2. (intransitive, dated, slang) To die.