Semantic satiation (also semantic saturation) is a psychological phenomenon in which repetition causes a word or phrase to temporarily lose meaning for the listener, who then processes the speech as repeated meaningless sounds.
Here's a fun experiment that you can do, right now, without leaving your computer. Take a two-word expression, any two words, let's say, "divine inspiration," and say it out loud repeatedly.
After the 20th or 30th time, the words will seem like a blur in your head. To an outsider, your words will make sense, but inside you will not be sure about what you are saying.