KO

from The American Heritage® Dictionary of the English Language, 5th Edition.

• transitive verb To knock out, as in boxing.
• noun A knockout, as in boxing.

from The Century Dictionary.

• An obsolete or dialectal contraction of quoth.
• noun A Chinese unit of customary enumeration, in connection with certain classes of things: as, yi ko jin, a man or ‘one man’ (usually rendered in pidgin-English ‘one piecey man’).
• noun An abbreviation of kathodic opening contraction.

from the GNU version of the Collaborative International Dictionary of English.

• noun acronym a knockout; a blow that renders the opponent unconscious; -- used especially in boxing.
• transitive verb acronym To knock out; to deliver a blow that renders (the opponent) unconscious; -- used especially in boxing.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun boxing A knock out.
• verb transitive (boxing) To knock somebody out, to render somebody unconscious.

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• noun a blow that renders the opponent unconscious