fes ‧ ter /ˈfestə $ -ər/ BrE AmE verb [intransitive]
[ Date: 1300-1400 ; Language: Old French ; Origin: festre 'sore hole in flesh' , from Latin fistula ]
1 . if an unpleasant feeling or problem festers, it gets worse because it has not been dealt with:
The dispute can be traced back to resentments which have festered for centuries.
2 . if a wound festers, it becomes infected:
festering sores
3 . if rubbish or dirty objects fester, they decay and smell bad