English, like many languages, changes over time. Not only do we add words, and drop others from everyday usage, but we also see existing words come to mean something different over time. If you watch old movies, you might be aware of the phrase “Bully for you!” That did not mean the speaker or the person spoken to was a bully.
10. Bully
Used to mean: Superb, wonderful
When Theodore Roosevelt referred to the presidency as a bully pulpit, he wasn't talking about name-calling, harassment, or beating anyone with a big stick. He was praising the social change he might shape in office. Bully for him!
11. Matrix
Used to mean: The womb
Morpheus was right. We've all lived in the Matrix.
Number 11 should make sense if you’ve read the Old Testament. Sacrificial offerings were supposed to be the “first fruits” of crops, or the livestock that “opens the matrix,” meaning the firstborn. Even if you knew these, there are probably some on the list of 15 words at mental_floss you didn’t realize once meant something very different.
12. DEFECATE
Used to mean: To purify something
From the Latin defæcatus, which translates to "cleanse from dregs," this definition still makes sense. Still, you'd probably decline if someone offered you a glass of defecated water.