Green’s Dictionary of Slang

(orig. US gay) a phr. implying amazement or shock, both approving and disapproving.

1968		‘Hy Lit’ Hy Lit’s Unbelievable Dict. of Hip Words 2: are you ready for this – setting the scene to wake everybody up on a very important happening.
1970		J.P. Stanley ‘Homosexual Sl.’ in AS XLV:1/2 45: The questionnaire was a list of terms known to the investigator and thought to have the greatest currency: trick, basket, box, camp, queen, Mercy! For days! Are you prepared?
1972		B. Rodgers Queens’ Vernacular 21: are you ready? means the same as ‘are you prepared to accept and believe this as a fact?’.