Readings & Meanings:
From the pros and cons, we could not get a conclusion.
"No!" he said as he grabbed his sword.
In the courtroom, he denied killing his wife.
否 for "no" is a samurai word, kind of old. You might hear it in samurai movies, but then again, as it's typically spat out in a rush, you might not catch it.
不 is straight from China; it negates over there, too. Add 不 to a word and you negate it. 不便 is the opposite of 便利.
口 is mouth.
So when you negate with your mouth, you say 否.