This is my bottom line — I can't make any more concessions.
Tips
usage
触碰底线 (chùpèng dǐxiàn, 'to touch the bottom line') means to cross someone's boundary. 守住底线 (shǒuzhù dǐxiàn, 'to hold the bottom line') means to maintain one's principles. These are extremely common phrases in both personal and political contexts.