讳莫如深

important matter that must be kept secret (idiom); don't breathe a word of it to anyone!

to avoid mentioning / taboo word / name of deceased emperor or superior

surname Mo

do not / there is none who

it would be better

as / as if / such as

old variant of 深[shen1]

deep / depth / deeply / (of a color) dark / deep / rich

deep (lit. and fig.)