▲ (OnlineEtymologyDictionary: silly )

silly (adj.)

Old English gesælig "happy, fortuitous, prosperous" (related to sæl "happiness"),
from Proto-Germanic *sæligas
(cognates: Old Norse sæll "happy,"
Old Saxon salig,
Middle Dutch salich,
Old High German salig,
German selig "blessed, happy, blissful,"
Gothic sels "good, kindhearted"),
from PIE *sele- "of good mood; to favor,"
from root *sel- (2) "happy, of good mood; to favor"
(cognates: Latin solari "to comfort,"
Greek hilaros "cheerful, gay, merry, joyous").

The word's considerable sense development moved
from "happy" to "blessed"  to "pious,"  to "innocent" (c. 1200),
to "harmless,"  to "pitiable" (late 13c.), "weak" (c. 1300),
to "feeble in mind, lacking in reason, foolish" (1570s).

Further tendency toward "stunned, dazed as by a blow" (1886) in knocked silly, etc. Silly season in journalism slang is from 1861 (August and September, when newspapers compensate for a lack of hard news by filling up with trivial stories). Silly Putty trademark claims use from July 1949.