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taboo word; forbidden topic
words or phrases not allowed (in waka and haikai)
その話題は彼の前では禁句だ。
That topic is a taboo subject in front of him.
面接で給与を先に尋ねるのは禁句とされる。
Asking about salary first in an interview is considered taboo.
禁句を口にして場の空気が凍りついた。
Uttering the forbidden topic froze the atmosphere.