unlucky number
A number considered unlucky or taboo, often due to cultural associations (e.g., 4 and 9 in Japan).
In Japan, 4 and 9 are often considered unlucky numbers.
Standard kanji spelling.
Abbreviated spelling; less common.
Compound of 忌み (imi, 'abhorrence, taboo') + 数 (kazu, 'number'). The exact historical derivation is uncertain, but the word transparently means 'taboo number'.