Silly entered English (spelled seely) in the 13th century from a Germanic source, in the senses ‘happy, blissful’ and ‘lucky, auspicious’. From this the senses ‘spiritually blessed’ and ‘pious and holy’ arose. A subtle development in meaning from ‘innocent, harmless’ (often referring to animals) to ‘deserving of pity’ led to ‘feeble’ and then, from the early 16th century, ‘foolish, simple’. The spelling silly is recorded from the 15th century.