/ krʌks; NAmE / noun
[ sing. ] the ~ (of sth) the most important or difficult part of a problem or an issue
SYN nub :
Now we come to the crux of the matter .
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WORD ORIGIN
mid 17th cent. (denoting a representation of a cross, chiefly in crux ansata ankh, literally cross with a handle): from Latin, literally cross .