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. Semantic drift, a linguistics term, is the way words change meaning in their lifespan. There are many ways they do this, which are processes called widening, narrowing, metaphor, metonymy, synendoche, degeneration, and a few others (though they have not all been decided upon in the linguistics community).
Silly is an example of degeneration–the gradual worsening of the meaning. At the beginning of the 13th century, silly was spelled “sely” and meant “happy, blissful, blessed.” It was derived from the noun, selth which meant “well-being” (as opposed to unselth which meant “misery”). The word took on more religious meanings as “happy” people were seen as “taken care of by good.” The meaning of sely thus narrowed into: “spiritually blessed,” which also implied “innocent.”
Now, there’s that other flip side of “innocence”. When you’re innocent, you need to be taken care of. You’re dependent and can’t think of yourself. You’re kind of… stupid. This was the birth of its modern meaning–beginning in the 1500s, sely was pronounced in the contemporary fashion as “silly” and meant: “lacking good sense, empty-headed, senseless, foolish.” Sound a little crazy?
Indeed, “this is the silliest stuff that ever I heard”(1595, Shakespeare, A Midsummer Night’s Dream).