Origin and history of especially

late 14c., from Old French especial "pre-eminent, important," from Latin specialis "belonging to a particular kind or species," from species "kind" (see species). Latin words with initial sp-, st-, sc- usually acquired an e- in Old French (see e-). Modern French has restored the word to spécial. In English, originally with the same sense as special (adj.), later restricted to feelings, qualities, etc.

late 13c., specialli, "of special purpose, for a special reason," from special (adj.) + -ly (2). As "more than others, above and beyond," mid-14c. A doublet of especially.

Special is more common than especial, which has the same meaning; but especially is for rhythmical reasons (because it occurs most frequently at the beginning of a dependent clause, where usually an unaccented particle occurs, and where, therefore, a word with an accent on the first syllable is instinctively avoided) much more common than specially. [Century Dictionary]

common adverbial suffix, forming, from adjectives, adverbs signifying "in a manner denoted by" the adjective; Middle English -li, from Old English -lice, from Proto-Germanic *-liko- (source also of Old Frisian -like, Old Saxon -liko, Dutch -lijk, Old High German -licho, German -lich, Old Norse -liga, Gothic -leiko). See -ly (1). It is 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.