Euphemism comes from Greek eúphēmos, which means "uttering sounds of good omen," "fair-sounding," or "auspicious." The first part of that root is the prefix eu-, meaning "good."  The second part is phēmos, a Greek word for "speech."

How and Why We Use Euphemisms

Euphemisms can take different forms, but they all involve substituting a word or phrase considered to be less offensive than another. The substituted word might, for example, be viewed as a less coarse choice, as when dang or darn is used instead of damn or damned. Or it might replace a word viewed as insulting to a religious figure, such as the various euphemisms for God (gad, gadzooks, gosh) or Jesus (gee, jeepers, jeez).A euphemism may also consist of an indirect softening phrase that is substituted for the straightforward naming of something unpalatable. Thus, we hear of people being “let go” rather than “fired”; civilians killed in war described as “collateral damage”; or someone who has died having “kicked the bucket,” “passed away, “given up the ghost,” or “joined one’s ancestors.”