A special source of synonymy is the so-called euphemism (eu - well, phemi -1 speak).
Euphemism is a way of speaking by which an unpleasant, improper _or offensive thing is designated by an indirect and milder term.
The roots of euphemisms lie in religious taboos which dictated the avoidance of certain terms, such as words connected with death, sacred beings, devil, etc. Such euphemisms are called superstitious taboos.
Superstitious taboos have their roots in the distant past of mankind when people believed that there was a supernatural link between a name and the object or creature it represented. Therefore, all the words denoting evil spirits, dangerous animals, or the powers of nature were taboo. If uttered, it was believed that unspeakable disasters would result not only for the speaker but also for those near him.
That is why all creatures, objects and phenomena threatening danger were referred to in a round-about descriptive way. So, a dangerous animal might be described as the one-lurking-in-the-wood, a mortal disease - the black death.
The Christian religion also makes certain words taboo. The fear of calling the devil by name (proverb «Speak of the devil and he will come») was inherited from ancient superstitious beliefs. So, the word devil became taboo, and a number of euphemisms began to substitute it: the Prince of Darkness, the black one, dickens (coll.), (Old) Nick (coll.).
Since the 16th century, when the use of the words god, Jesus and the Trinity was forbidden in profane language many synonymic substitutes to designate these ideas were developed, e.g. instead of god - cock, cot, gog, gosh, goodness, goodness gracious, Lord, the Maker, Good, by Heavens.
Even in modern times most people are reluctant to use the verb to die. It has the following euphemisms: to pass away, to be taken, to breathe one's last,
to depart this life, to close one's eyes, to go the way of all flesh, to go West (si.), to kick off (si.), to kick the bucket (si.), to join the majority, to go to another world.
In the insurance companies people write: «If anything should happen to me...» (if I should die).
Instead of dead it is common to say: the departed, the deceased, the late Mr. Smith, etc.