Where does “backwards compatibility” come from?

backwards compatibility (English) comes from English compatibility, from English compatible, from Middle English compatible, from Anglo-Norman compatible, from Medieval Latin compatibilis, from Late Latin compator, from Latin com-, from Latin con- — whale, sea monster; abyss.

Ancestry of “backwards compatibility”, step by step

backwards compatibility traces back through two separate lines of ancestry.

via English compatibility

StepLanguageWordMeaning
1EnglishcompatibilityThe state of being compatible; in which two or...
2EnglishcompatibleCapable of easy interaction; Able to get along...
3Middle Englishcompatiblecompatible; reconcilable
4Anglo-Normancompatible
5Medieval Latincompatibilisin "compatible beneficium", a benefice which...
6Late Latincompator
7Latincom-allomorph of con-
8Latincon-Used in compounds to indicate a being or bringing...
9Latincumwith, along with; at; -fold
10Old LatincomAlternative form of cum
11Proto-Italickomwith, along, at
12Proto-Indo-Europeanḱómbeside, near, by, with
13Proto-Indo-EuropeanḱeDeictic particle, here; Post-positional...
14Middle Englishkyt
15Russianкитwhale; genitive plural of ки́та
16Old Church Slavonicкитъwhale
17Ancient Greekκῆτοςwhale, sea monster; abyss

via English backwards

StepLanguageWordMeaning
1EnglishbackwardsOriented toward the back; Reversed; Behind...
2English-sUsed to form regular plurals of nouns; Used to...
3English’s
4EnglishKingstownOS grid ref NY3959
5EnglishtownA settlement; an area with residential districts,...
6Middle Englishtountown
7Middle EnglishendeThe end or finishing of a thing; the terminal...
8Old Englishendeend, limit, border, corner
9Proto-Germanicandiin addition, furthermore, and
10Proto-Indo-Europeanh₂éntiopposite; near; in front
11Proto-Indo-Europeanh₂éntsforehead; front
12Proto-Indo-Europeanh₂ent-face; forehead; front
13Proto-Indo-Europeanh₂en-on, onto
Every word from Ancient Greek κῆτος