A misspelling or rare variant of 'decrepitude,' sometimes used as a folk etymology.
Appears to be a conflation or misspelling of 'decrepitude,' possibly influenced by folk etymology connecting it to 'crept' (past tense of creep). This form is not standard in modern usage.
Interestingly, 'decreptitude' shows how people sometimes mishear or mishape words over time—it's a linguistic fossil from when people tried to make sense of an old word!
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