pejoration

[pej-uh-rey-shuh n, pee-juh-] /ˌpɛdʒ əˈreɪ ʃən, ˌpi dʒə-/
noun
1.
depreciation; a lessening in worth, quality, etc.
2.
Historical Linguistics. semantic change in a word to a lower, less approved, or less respectable meaning.
Compare melioration (def 1).
Origin
1650-60; < Medieval Latin pējōrātiōn- (stem of pējōrātiō) a making worse, equivalent to Late Latin pējōrāt(us) (past participle of pējōrāre to make worse, derivative of pējor worse) + -iōn- -ion
British Dictionary definitions for pejoration

pejoration

/ˌpiːdʒəˈreɪʃən/
noun
1.
(linguistics) semantic change whereby a word acquires unfavourable connotations: the English word ``silly'' changed its meaning from ``holy'' or ``happy'' by pejoration Compare amelioration (sense 3)
2.
the process of worsening; deterioration
Word Origin and History for pejoration
n.

1650s, noun of action from pejorate (see pejorative).