Middle English redi, with adjectival suffix -i (as in busy, crafty, hungry, etc.) + Old English ræde, geræde "prepared, ready, suitably equipped;" of a horse, "ready for riding."
This is reconstructed to be from Proto-Germanic *(ga)raitha- "arranged" (source also of Old Frisian rede "ready," Middle Dutch gereit, Old High German reiti, Middle High German bereite, German bereit, Old Norse greiðr "ready, plain," Gothic garaiþs "ordered, arranged"), which is perhaps from PIE root *reidh- "to ride" (see ride (v.)).
Lengthened in Middle English by change of ending. Sense of "at hand, present, available" is late 12c. Of money, "immediately available," c. 1300, hence slang noun the ready "cash" (1680s). Phrase at the ready "in the position of a soldier's firearm after the command '(make) ready!'" is attested from 1837. As an adverb, c. 1300, "at hand." A ready-reckoner (1757) was a book of tabulated calculations of the sort used in ordinary business and housekeeping.
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.