rub
US /rʌb/
UK /rʌb/

1.
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2.
症结, 困难
a difficulty or problem, especially one that is hidden or not immediately obvious
:
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The only rub is that we don't have enough money.
唯一的症结是我们没有足够的钱。
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Here's the rub: we need to finish this by tomorrow.
症结在于:我们明天之前必须完成这个。