Where does “needn't've” come from?

needn't've (English) comes from English needn’t, from English -n’t, from English Not, from Middle English not, from Middle English nought, from Old English nōwiht, from Old English ne, from Old English ealles — Creates action nouns or result nouns from verbs;...

Ancestry of “needn't've”, step by step

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

via English needn’t

StepLanguageWordMeaning
1Englishneedn’tNeed not
2English-n’t
3EnglishNot
4Middle Englishnotnot; nothing, nought; nobody, no person
5Middle Englishnoughtnothing, none; not; iniquitous, wicked
6Old Englishnōwiht
7Old Englishnenot; or, and not; "ne... ne..." is used to mean "...
8Old Englisheallescompletely, wholly, entirely; totally,...
9Old English-espossessive marker, indicating that an object...
10Proto-Germanic-asused in formation of adverbs
11Proto-Indo-European-ēsPrimarily forms adjectival bahuvrīhi compounds
12Proto-Indo-European-ess
13Proto-Indo-European-osCreates action nouns or result nouns from verbs;...

via English ’ve

StepLanguageWordMeaning
1English’vein its sense marking the perfect or retrospective aspect
Every word from Proto-Indo-European -os