Calling a Jaguar XK a jalopy, say

Wikipedia Word definitions in Wikipedia
A dysphemism is an expression with connotations that are offensive either about the subject matter or to the audience, or both. Dysphemisms contrast with neutral or euphemistic expressions. Dysphemism is sometimes motivated by feelings such as fear, distaste, ...

WordNet Word definitions in WordNet
n. an offensive or disparaging expression that is substituted for an inoffensive one; "his favorite dysphemism was to ask for axle grease when he wanted butter" [ant: euphemism ]

Wiktionary Word definitions in Wiktionary
n. 1 (context uncountable English) The use of a derogatory, offensive or vulgar word or phrase to replace a (more) neutral original. 2 (context countable English) A word or phrase that is used to replace another in this way.

Douglas Harper's Etymology Dictionary Word definitions in Douglas Harper's Etymology Dictionary
1884, "substitution of a vulgar or derogatory word or expression for a dignified or normal one," from Greek dys- "bad, abnormal, difficult" (see dys- ) + pheme "speaking," from phanai "speak" (see fame (n.); Greek dysphemia meant "ill language, words of ...