silliness n.

Middle English seli, seely, from Old English gesælig "happy, fortuitous, prosperous" (related to sæl "happiness"), from Proto-Germanic *sæligas (source also of Old Norse sæll "happy," Old Saxon salig, Middle Dutch salich, Old High German salig, German selig "blessed, happy, blissful," Gothic sels "good, kindhearted").

This is one of the few instances in which an original long e (ee) has become shortened to i. The same change occurs in breeches, and in the American pronunciation of been, with no change in spelling. [Century Dictionary]

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

It is a widespread phenomenon that the words for 'innocent', apart from their legal use, develop, through 'harmless, guileless', a disparaging sense 'credulous, naive, simple, foolish.' [Buck]

There may be a further specialization toward "stunned, dazed as by a blow" (1886) as in knocked silly, etc. As a noun, "a silly person," by 1858 in writing for children.

Silly season in journalistic slang is from 1861 (in reference to August and September, when newspapers compensate for a lack of hard news by filling up with trivial stories). The trademark for the toy Silly Putty claims use from July 1949. Sillyism "a silly statement or utterance" is from 1706.

word-forming element denoting action, quality, or state, attached to an adjective or past participle to form an abstract noun, from Old English -nes(s), from Proto-Germanic *in-assu- (cognates: Old Saxon -nissi, Middle Dutch -nisse, Dutch -nis, Old High German -nissa, German -nis, Gothic -inassus), from *-in-, originally belonging to the noun stem, + *-assu-, abstract noun suffix, probably from the same root as Latin -tudo (see -tude).