gruntle

The verb disgruntle, which has been around since 1682, means "to make ill-humored or discontented." The prefix dis- often means "to do the opposite of," so people might naturally assume that if there is a disgruntle, there must have first been a gruntle with exactly the opposite meaning. But dis- doesn't always work that way; in some rare cases it functions instead as an intensifier. Disgruntle developed from this intensifying sense of dis- plus gruntle, an old word (now used only in British dialect) meaning "to grumble." In the 1920s, a writer humorously used gruntle to mean "to make happy"—in other words, as an antonym of disgruntle. The use caught on. At first gruntle was used only in humorous ways, but people eventually began to use it seriously as well.