What's cooking? asked a friend!
I blurted out, "I am not cooking anything,"
and we ended up laughing so hard.
Many of the expressions that we regard as slang or colloquial usage today were "old stuff" to the Greeks and Romans thousands of years ago. They had words for them--single words that no longer have any slang overtones for us today. We use the word concoct without realizing that in one dignified word we are echoing the slang phrase, "to cook up." Yet that is what concoct mean literally (coct from the Latin verb meaning "to cook" + con, together), and the obvious question to ask someone who is concocting something is, "what's cooking?"