Silly, a. [Compar. Sillier; superl. Silliest.]
Etymology: [OE. seely, sely, AS. s, ges, happy, good, fr. s, s, good, happy, s good fortune, happines; akin to OS. salig, a, good, happy, D. zalig blessed, G. selig, OHG. salig, Icel. s, Sw. säll, Dan. salig, Goth. s good, kind, and perh. also to L. sollus whole, entire, Gr. sarva. Cf. Seel, n.]
1. Happy; fortunate; blessed. [Obs.] Chaucer.
2. Harmless; innocent; inoffensive. [Obs.] "This silly, innocent Custance." Chaucer. The silly virgin strove him to withstand. Spenser. A silly, innocent hare murdered of a dog. Robynson (More's Utopia).
3. Weak; helpless; frail. [Obs.] After long storms . . . With which my silly bark was tossed sore. Spenser. The silly buckets on the deck. Coleridge.
4. Rustic; plain; simple; humble. [Obs.] A fourth man, in a sillyhabit. Shak. All that did their silly thoughts so busy keep. Milton.
3 more senses
5. Weak in intellect; destitute of ordinary strength of mind; foolish; witless; simple; as, a silly woman.
6. Proceeding from want of understanding or common judgment; characterized by weakness or folly; unwise; absurd; stupid; as, silly conduct; a silly question.
Syn. -- Simple; brainless; witless; shallow; foolish; unwise; indiscreet. See Simple.
Shades of meaning
Fernald lists silly only as an antonym, under sagacious.
Where he writes about it:
Anything is irrational when clearly contrary to sound reason, foolish when contrary to practical good sense, silly when petty and contemptible in its folly, erroneous when containing error that vitiates the result, unreasonable when there seems a perverse bias or an intent to go wrong. — from absurd