Stephen Colbert, in his 7/25/18 monologue, uttered this one when discussing the Cohen tape on the Trump payoff of the McDougall matter. Here’s the link to the video:
It is a mashup of “in the heat of the moment” (doing something without thinking) and “the meat of the matter” (the most important or essential element of an issue or problem). “Heart of the matter” (same definition as “meat of the matter”) might also be in play, as “heart” and “heat” are similar in sound and looks. A big thanks to Sam Edelmann for hearing this one and sending it in.