Euphemism

Euphemism: (From the Greek,"good speech") A more agreable or less unpleasant substitute for a generally unpleasant word or concept.

Example:
" Shucks, it ain't no use to talk to you, Fuck Finn. You don't seem to know anything, somehow-perfect sap-head."
                                       (Huckleberry Fin, Pg.14)

This is euphemism because Jim calls Huck a "Sap-head." That could mean anything he might of tried and called him "hard headed" or something else but instead he calls him a "sap-head."