sillily (adv.)

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.

common adverbial suffix, forming from adjectives adverbs signifying "in a manner denoted by" the adjective, Middle English, from Old English -lice, from Proto-Germanic *-liko- (cognates: Old Frisian -like, Old Saxon -liko, Dutch -lijk, Old High German -licho, German -lich, Old Norse -liga, Gothic -leiko); see -ly (1). Cognate with lich, and identical with like (adj.).

Weekley notes as "curious" that Germanic uses a word essentially meaning "body" for the adverbial formation, while Romanic uses one meaning "mind" (as in French constamment from Latin constanti mente). The modern English form emerged in late Middle English, probably from influence of Old Norse -liga.