Add cold water and ground coffee to the big jar and mix really well with the whisk. Leave the lid open for 10 minutes to let some CO2 out. (Natural CO2 gas is released from coffee when mixed with water and especially when the coffee is very fresh)
Close lid and let sit at room temp for 20-24 hours.
Strain through a fine mesh strainer and store in glass bottles in the fridge.
Set it in the fridge for up to 4 weeks. Serve over lots of ice and dilute with splash of milk, oat milk, almond milk, coconut milk or water because it's very strong.