The present day English word silly is not especially ambiguous, and we are generally familiar with its use in the sense 'foolish, senseless or stupid'. However, silly has not always been defined in this way, and by looking into its history, we can uncover a variety of different meanings and observe a curiously elaborate process of semantic change.
Silly is first recorded in Old English, in the form sælig, with the senses 'fortuitous, happy or prosperous'. The sense 'spiritually blessed' is also attested early, as is the sense 'pious, holy, and good'. In the early thirteenth century there are textual references to 'holy martyrdom' as seli martyrdom and a female saint is referred to as a seli meiden or 'blessed maiden'. Similarly, a prayer book from the early fifteenth century describes the cely or 'blessed' Virgin Mary as follows:
Cely art thou, hooli virgyne marie, and worthiest al maner preisyng.
During the Middle English period, and especially from the late thirteenth century onwards, the meanings of the word silly become more diverse. At this time, the sense 'blessed or holy' develops into a new sense, 'innocent', but this in turn provokes a number of more negative senses, including 'harmless', 'deserving of pity', 'helpless', 'insignificant' and 'feeble'.
Further developments take place in early modern English. In the sixteenth century, the senses 'deserving of pity' and 'helpless' give rise to new interpretations of silly as 'unlearned, unsophisticated, simple, rustic and ignorant', which leads to the use of the word to denote someone who is 'humble in rank' or 'lowly'. Holinshed's Chronicles, for example, record this use of the word in the description an incident where a sillie footman shoots a man of higher social status. Also in the sixteenth century, the earlier sense 'feeble' gives rise to the meaning 'frail, weakly, sickly, worn-out or crazy', and the various senses 'feeble', 'weak' and 'helpless' all contribute to the use of silly meaning 'deficient in intellect, feeble-minded'.
The modern sense 'lacking in judgement or common sense, foolish' also appears for the first time in the sixteenth century, and eventually becomes the dominant usage. From our brief history of the word, we can see elements of many of the earlier senses at work in the current definition, including 'feeble-minded', 'simple' and 'ignorant', and we can also trace the development of these meanings back over the last millennium to a time when being 'silly' had entirely different connotations.