Silly means foolish in a light, childish way. A silly question, don't be silly, a silly hat. It is affectionate far more often than it is critical, which is what separates it from stupid.
That difference matters. Calling a person stupid is an insult; calling them silly is usually teasing, and don't be silly is said to reassure somebody who is worrying about nothing. Applied to a decision or an amount, silly means unreasonable — a silly price, silly o'clock in the morning. Note the comparative silliest and the noun silliness. In cricket, silly is a fielding position dangerously close to the batter, which is exactly what it sounds like.
1adjectivesillyFoolish in a light or childish way.
“Do not be silly, of course you can come.”
2adjectivesensibleNot sensible or reasonable.
“They are asking a silly price for it.”
3adjectiveplayfulPlayful and not serious.
“It was a silly game and everybody enjoyed it.”
Too few real examples of this word to estimate which sense is commonest.
silly is a degree of foolishness.
Also accepted in games: foolish
Old English saelig meant happy and blessed. It drifted through innocent, then harmless, then simple, and finally to foolish — a thousand-year slide from blessed to daft that is one of the best-documented meaning changes in English.
Proto-West Germanic
*sālīg
Old English
sǣliġ
Middle English
seely
English, today
silly
Built from
From Middle English seely, seli, from Old English sǣliġ, ġesǣliġ (“lucky, fortunate”), from Proto-West Germanic sālīg, from sāli; equivalent to seel + -y. a doublet of Seelie. The semantic evolution is “lucky” to “innocent” to “naive” to “foolish”. Compare the similar evolution of daft (originally meaning “accommodating”), and almost the reverse with nice (originally meaning “ignorant”).
Etymology from Wiktionary, CC BY-SA