Where does “you'dn't've” come from?

you'dn't've (English) comes from English you, from Middle English ȝow, from Old English ēowu, from Proto-Germanic awiz, from French avis, from Middle French advis, from Dutch advies, from Middle Dutch avijs.

Ancestry of “you'dn't've”, step by step

you'dn't've traces back through two separate lines of ancestry.

via English you

StepLanguageWordMeaning
1EnglishyouThe people spoken, or written to, as an object;...
2Middle EnglishȝowAlternative form of yow
3Old Englishēowuewe
4Proto-Germanicawizewe; sheep in general
5Frenchavisopinion; piece of advice; notice
6Middle Frenchadvisopinion
7Dutchadviesadvice
8Middle Dutchavijs
9Old Frenchavisopinion
10Old Frenchato; towards; belonging to
11LatinA
12Etruscan𐌀and, also
13Ancient Greekᾱ̔
14Phoenician𐤀ox; aleph; alef
15Egyptian𓃾

via English wouldn't

StepLanguageWordMeaning
1Englishwouldn'twould not
2English-n'tNegates the meaning of the clause in which it...
3EnglishnotNegates the meaning of the modified verb; To no...
4Middle EnglishnatAlternative form of not
5Middle EnglishnoghtAlternative form of nought
6Old Englishnāhtnought, nothing
7Old Englishnōwiht
8Old Englishnenot; or, and not; "ne... ne..." is used to mean "...
9Old Englisheallescompletely, wholly, entirely; totally,...
10Old English-espossessive marker, indicating that an object...
11Proto-Germanic-asused in formation of adverbs
12Proto-Indo-European-ēsPrimarily forms adjectival bahuvrīhi compounds
13Proto-Indo-European-ess
14Proto-Indo-European-osCreates action nouns or result nouns from verbs;...
Every word from Egyptian 𓃾Every word from Phoenician 𐤀Every word from Ancient Greek ᾱ̔