If you walked into a restaurant in Madras (now called Chennai) in India and ordered a “Madras” you’d almost certainly be met with a blank look. It’d be the same as walking into a restaurant in the English capital and asking for a “London”.
The Madras is a British invention and its connotations with “hot” stem from the traders and soldiers who were in the city from the time the British arrived in 1640. Not only do South Indians love spicy food but the city is extremely hot and humid, with temperatures usually over 30°C (86°F) and frequently reaching 40°C (104°F).
Those early ex-pats would have brought back the tastes of India when they returned home with their pots of spice mixes, or early curry powders. As there were no standard for these spice mixes (just as not all curry powders are the same today), it’s possible that the mixes with a little bit extra zing were called “Madras” to acknowledge their extra heat.
The early Indian restaurant owners in Britain carried through this thinking by adding their own hotter mixes or more chilli powder to their standard curry to create the Madras Curry and why today many people are able to order virtually any dish on the menu and ask the chef to make it “Madras hot”.