The phrase that follows - "dull hues of so many everyday words" - explains in further detail what the writer means by "slack." "Slack" means "not tight," such as a slack or loose rope that needs to be tightened in order to hold something securely. The author implies that words used in everyday conversation have lost their "freshness" or power, because they are heard so commonly.

"Slack" here means "loose", the opposite of taut. "Slack, dull hues" is a metaphor describing the fact that commonly spoken words don't have the same power to make you visualize and immerse yourself in the same way literary writing style does.