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.
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).
<a href="https://www.etymonline.com/word/readiness">Etymology of readiness by etymonline</a>
Harper, D. (n.d.). Etymology of readiness. Online Etymology Dictionary. Retrieved $(datetime), from https://www.etymonline.com/word/readiness