To engage in secret, often illicit, dealings or relationships; to collude in an underhand way; to carry on a clandestine affair.
那两个人最近总是勾勾搭搭的，大家都在猜测他们是不是有什么不可告人的计划。
Those two have been secretly colluding lately, and everyone is guessing if they have some ulterior motive.
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