A nonstandard or dialectal form attempting to represent 'was not,' though the correct modern contraction is 'wasn't'.
From the verb 'was' (past tense of 'be') combined with 'not'; like other incomplete contractions, 'wasn' never standardized while 'wasn't' became the accepted form through conventional use.
The 'n't' in modern contractions is actually the ghost of the word 'not'—you can see it struggling to attach to verbs in forms like 'wasn,' showing that English speakers have been smashing words together for centuries.
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