nub
noun /nʌb/
  /nʌb/
[singular]- the nub (of something) the central or essential point of a situation, problem, etc.中心;要点;实质 - The nub of the matter is that business is declining.问题的核心是营业额在萎缩。 
- She’s too selfish to help. That’s the nub.她太自私了,不愿帮忙。这才是重点。 
 词源late 17th cent.: apparently a variant of dialect knub ‘protuberance’, from Middle Low German knubbe, knobbe ‘knob’.
- The nub of the matter is that business is declining.