Instead of being agreeable and amiable, I (or he, or she, or they) will be tough and uncompromising. This colloquialism dates from the mid-1900s, and is a modern, far more benign substitute for grant no quarter. It appears in numerous contexts and sometimes does not even involve a person, as in “The weatherman has stopped playing Mr. Nice Guy,” which simply means he is announcing a change to bad weather. Lawrence Block used it with more dramatic meaning in All the Flowers Are Dying (2005): “‘Get three [guns],’ she said. ‘One for each of us. No more Mr. Nice Guy.’”