They sacked down for some romantic time.
What does 'sacked down' mean here?
It's some kind of rarely used slang, and probably means to go to bed for an amorous purpose. I'd avoid using it if I were you; most native speakers don't use it, and would have to guess at its meaning (as I'm now doing!).
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It sounds to me like BrE, but to my American ears, I would assume it means to get into bed.
We have an idiom "To hit the sack", which means to go to sleep.
Yes, it sounds like a hybrid of "hit the sack" and something like "doss down," which I've only read in British novels. I've never heard "sack down" in my life. It certainly doesn't sound very romantic.