/ ˈfestə(r); NAmE / verb [ v ]
1.
( of a wound or cut ) to become badly infected :
festering sores / wounds
2.
( of bad feelings or thoughts ) to become much worse because you do not deal with them successfully
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WORD ORIGIN
late Middle English : from the rare word fester fistula , later festering sore , or Old French festrir (verb), both from Old French festre (noun), from Latin fistula pipe, reed, fistula.