Words pass through time like great works of art or literature. Rarely do they receive the same interpretations they did in their inception. The process driving their evolution is semantic drift, the way words change meaning in their lifespan.
Semantic Drifting: The Story of the Word “Silly”
Did you know the girl used to refer to both sexes? Or that awful meant even more awesome than awesome (some awe vs. full of awe)? Your brain in a choke hold right now? You can thank semantic drift.