Imi-kotoba are words that are avoided in certain situations because they are thought to bring bad luck. When one of these words must be used, the speaker either uses an alternate pronunciation, or a “safe” substitution word.
In Japanese, the number four has two pronunciations: yon and shi. Shi is also the pronunciation of the character 死, which means “death.” A superstitious person will resolutely avoid using the shi pronunciation of the number four. A mildly superstitious person will avoid using it in important or potentially dangerous situations.
The most common Japanese word for pear is nashi 梨. Nashi, however, is also the pronunciation of 無し, a word which means “none.” To avoid the hazards of this association, the word ari-no-mi / 有りの実can also be used to refer to a pear. Ari-no-mi has a much more auspicious etymology; its literal translation would be “the fruit of abundance.”
As mentioned above, the imi-kotoba are most feared on momentous occasions. Wedding guests, for example, will avoid using the word kireru / 切れるin any context. Kireru means “to be severed” or “to be cut.” To the superstitious, the dire implications of using such a word at a marriage celebration will be obvious.