1Deterioration and depreciation of property caused by changes made nearby by a person or group other than the owner, e.g. a neighbour or local authority.
Frequently (sometimes as a lawyers' colloquialism) in contrast with betterment; in earlier use often with reference to municipal road improvement schemes.
2Generally. The action of worsening something or an instance of this.
In most cases probably recoined each time as a nonce-use.
Late 19th century. Probably partly from worse + -ment, and partly from worse + -ment, after betterment.