This happens due to neural fatigue, where repeated firing of the brain's neurons connecting the word to its concept weakens the association, similar to how one gets used to a constant smell. It's also known as verbal satiation or semantic saturation and can occur with both spoken and written words.

Semantic satiation is a psychological phenomenon where repeating a word or phrase rapidly makes it temporarily lose its meaning, causing it to sound like a strange collection of sounds rather than a meaningful unit.