(with "the") Those who have been killed.
To kill, murder.
Our foes must all be slain.
To eradicate or stamp out.
You must slay these thoughts.
(by extension) To defeat, overcome (in a competition or contest).
1956, “Giants Slay Bears in Pro Title Battle”, in Lodi News-Sentinel, 1956 December 31, page 8.
To delight or overwhelm, especially with laughter.
Ha ha! You slay me!
To amaze, stun or otherwise incapacitate by awesomeness; to be awesome at something; to kill (slang sense).
To have sex with