The phrase made me think of wearing a loosely fitted garment, something allowing you to move easily. But theidioms says the phrase comes from docking ships, “where ‘give me some slack’ meant to loosen the rope.”
25-startling-origins doesn’t live up to its click-bait title: “It is believed to be nautical in origin, and concerns not pulling on the rope so as to give the other person a chance to untangle it.”
I’m more familiar with the version “cut me some slack,” and cutting the rope doesn’t sound like a good idea when docking a ship – though it may be a last resort to a tangled mess. But I admit I’m a landlubber. Someone on wordwizard who claims to know boats says “CUT SOME SLACK and CUT SOMEONE SOME SLACK are not nautical terms nor have they ever been!” They’re pretty emphatic about it.
The only citation I found was this:
A similar phrase, with a similar meaning but slightly different form – ‘cut slack for’ – was used in 1855 by Frederick Douglas in his book My Bondage and My Freedom. theidioms