Persistently present or troubling; constantly affecting or influencing someone.
From 'beset' + '-ing' (present participle/gerund suffix). The phrase 'besetting sin' became famous through Puritan religious literature, where it meant a sin that particularly afflicts an individual.
The phrase 'besetting sin' from Puritan theology became so famous that it entered common English—now people use 'besetting problem' or 'besetting weakness' without thinking about its religious origins!
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