From Wikipedia:
"'MP4' has been the chassis designation for all McLaren Formula 1 cars since 1981. Since 1997, 'MP4' has stood for McLaren Project 4 (from 1981 to 1996, when McLaren's title sponsor was
Marlboro, the "M" stood for this), resulting from the merger of
Ron Dennis' Project 4 organisation with McLaren. The '12' refers to McLaren's internal Vehicle Performance Index through which it rates key performance criteria both for competitors and for its own cars. The criteria combine power, weight, emissions, and aerodynamic efficiency. The coalition of all these values delivers an overall performance index that has been used as a benchmark throughout the car's development. The 'C' refers to Carbon, highlighting the application of carbon fibre technology to the future range of McLaren sports cars."