positively adv.

early 14c., originally a legal term meaning "formally laid down, decreed or legislated by authority" (opposed to natural),  from Old French positif (13c.) and directly from Latin positivus "settled by agreement, positive" (opposed to naturalis "natural"), from positus, past participle of ponere "put, place" (see position (n.)).

The sense of "absolute" is from mid-15c. Meaning in philosophy of "dealing only with facts" is from 1590s. Sense broadened to "expressed without qualification" (1590s), then, of persons, "confident in opinion" (1660s). The meaning "possessing definite characters of its own" is by 1610s. The mathematical use for "greater than zero" is by 1704. Psychological sense of "concentrating on what is constructive and good" is recorded from 1916. Positive thinking is attested from 1953. The sense in electricity is from 1755.  

There are probably no two bodies differing in nature which are not capable of exhibiting electrical phaenomena, either by contact, pressure, or friction ; but the first substances in which the property was observed, were vitreous and resinous bodies ; and hence the different states were called states of resinous and vitreous electricity ; and resinous bodies bear the same relation to flint glass, as silk. The terms, negative and positive electricity, have been likewise adopted, on the idea, that the phaenomena depend upon a peculiar subtile fluid, which becomes in excess in the vitreous, and deficient in the resinous bodies ; and which is conceived by its motion and transfer, to produce the electrical phaenomena. [Sir Humphry Davy, "Elements of Chemical Philosophy," London, 1812]

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.