"modify so as to incorporate changes made in later versions of the same model," 1954 (U.S. Air Force), from retro- + fit (v.). Related: Retrofitted; retrofitting. As a noun, "modification made to a product," 1956, from the verb.
word-forming element of Latin origin meaning "backwards; behind," from Latin retro (prep.) "backward, back, behind," usually in reference to place or position, rarely of time, "formerly, in the past," probably originally the ablative form of *reteros, based on re- "back" (see re-).
L. retro stands to re- as intro, "in, within"; to in, "in," and as citro, "hither," stands to cis, "on this side." [Klein]
Common in combinations in post-classical Latin (the classical equivalent was post-). Active in English as a word-forming element from mid-20c.
c. 1400, "to marshal or deploy (troops);" early 15c. as "be fitting or proper, be suitable," from fit (adj.) and perhaps in part from Scandinavian (compare Old Norse fitja "knit"). From 1580s as "be the right shape." Transitive sense of "provide with what is suitable" is from 1590s; that of "make fit or suitable, bring into corresponding form or condition" is from c. 1600. Related: Fitted; fitting. Fitted sheets is attested from 1948.
<a href="https://www.etymonline.com/word/retrofit">Etymology of retrofit by etymonline</a>
Harper, D. (n.d.). Etymology of retrofit. Online Etymology Dictionary. Retrieved $(datetime), from https://www.etymonline.com/word/retrofit