noun
unlucky number
A number considered unlucky or taboo, often due to cultural associations (e.g., 4 and 9 in Japan).
日本では4と9が忌み数とされることが多い。
In Japan, 4 and 9 are often considered unlucky numbers.
縁起 refers to luck or omen in general, while 忌み数 specifically means an unlucky number.
Compound of 忌み (imi, 'abhorrence, taboo') + 数 (kazu, 'number'). The exact historical derivation is uncertain, but the word transparently means 'taboo number'.