Refers to a tooth with a cavity or decay. Often used in everyday conversation, though 虫歯 (むしば) is more common.
See also: 虫歯
My decayed tooth hurt, so I went to the dentist.
Compound of 虫食い (むしくい, 'worm-eaten') and 歯 (は, 'tooth'), literally 'worm-eaten tooth'. The exact historical derivation is uncertain, but the term vividly describes a tooth damaged by decay.