A nonstandard past tense form of backslide (the standard past tense is 'backslid').
Formed by adding the regular '-ed' suffix to 'backslide,' but this is nonstandard English—'backslid' and 'backslidden' are the correct past tense forms, following the irregular verb pattern of 'slide.'
Even educated speakers sometimes say 'backslided' instead of 'backslid,' which shows how strongly English speakers expect verbs to add '-ed' for past tense—it's a battle between regular and irregular patterns!
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