cook up
To manufacture; to fabricate; to falsify; to devise an elaborate lie.
To prepare a heroin dose by heating.
To prepare a meal.
To manufacture a significant amount of illegal drugs (LSD, meth, etc.)
cook off
To pull the pin from a grenade and wait two or three seconds before throwing.
As with above, except to unintentionally wait so long that the grenade detonates.
To cause an accidental detonation of explosives, especially due to excess heat.
To accidentally detonate, especially as the result of excess heat.