What does a snakeskin have in common with a swamp? (Aside from some snakes living in swamps.) The answer is slough, one of those odd words whose pronunciation determines its meaning and whose path to modern English is the convergence of two distinct words of Germanic origin.
Slough, pronounced like slew, refers to a marshy place or mire. As a verb, it can mean to plod through mud or for a slough to engulf something. The word’s origins are appropriately murky. It derives from Middle English sloughe or slo, which comes from Old English sloh, meaning muddy ground. After that the origins are uncertain. It could derive from Middle High German slouche, meaning ditch, or from Proto-Germanic *slokhaz.
Figuratively, slough describes a state of moral degradation. John Bunyan made use of this meaning in The Pilgrim’s Progress with the “Slough of Despond,” a deep bog into which Christian sinks under the weight of sin and guilt.
Then I stepped to him [Help] that pluckt him [Christian] out, and said, Sir, wherefore, since over this place is the way from the City of Destruction to yonder Gate, is it that this plat is not mended, that poor travellers might go thither with more security? And he said unto me, This miry Slough is such a place as cannot be mended; it is the descent whither the scum and filth that attends conviction for sin doth continually run, and therefore it is called the Slough of Dispond; for still as the sinner is awakened about his lost condition, there ariseth in his soul many fears and doubts, and discouraging apprehensions, which all of them get together, and settle in this place: And this is the reason of the badness of this ground (The Pilgrim’s Progress, The First Part, paragraph 53).
Slough, pronounced like sluff, means to shed or cast off skin, to separate dead tissue from living tissue. As a noun, it designates the dead tissue that has been cast off, particularly snake skin. The modern word derives from the Middle Englishslughe or slouh, which means the skin a snake has shed. The Middle English words are related to the Old Scandinavian sluk and Middle High German sluch, both of which mean snakeskin.
It’s fascinating that words with such different meanings would converge on the same spelling but retain different pronunciations. Here’s a fun exercise. Use the word slough in a sentence multiple times touching on its different meanings. Then read the sentence aloud, matching the pronunciation with the meaning.
For example: The snake was sloughed while sloughing in a slough. [Read as: The snake was slewed while sluffing in a slew.]