I want to take one last opportunity to mention the importance of the examples of word use given in the OED. Even students who have gotten terrific at using the OED tend to ignore them, but they are invaluable.
Here's the line. In today's assignment, Prufrock is remembering a time when he was still able to imagine himself being "wreathed with seaweed red and brown" by "sea girls" (the mermaids mentioned in the lines just above).
In ancient Greece, bestowing a laurel wreath on the winner of an athletic or poetic competition "crowning the victor"was an important symbolic occasion. (In Britain, a "poet laureate" is still chosen. "Laureate" means "crowned with laurel," a plant considered sacred by the ancient Greeks. Tennyson, for instance, whose Ulysses we read, was poet laureate under Queen Victoria.)
Now back to Prufock. As we've seen, he's had many moments in his earlier life when he was able to imagine himself declaring his love to the lady he worships and adores from afar, and when he could still fantasize about her responding with equal passion.
But here, at the end of the poem, he has finally come to realize that he will never dare to declare his passion to the lady"And would it have been worthwhile, after all," he asks himself, showing that he has given up his hopesand that he is doomed to grow old and die without ever really having lived.
The "mermaids" appear here because his unfulfilled conquest over his own fears and inward paralysis has now begun to appear to him as a kind of myth or fairy-tale. It was only earlier, when he was still able to imagine his success, that he was able to imagine himself being crowned or wreathed like the victor in a competition.
This is where the OED, as always, comes to the rescue. If you've been paying close attention to Prufrock's inward twistings and turnings, you'll have figured out that he's finally given up on himself ("would it have been worth while," etc). Once you've seen that the sea-girls represent a recent past that is even now receding into mere fantasy, in turn, you'll also see that he's talking about those earlier moments when he was still able to imagine himself (figuratively) crowned as a victor over his fears and self-doubts. Now, if you go to wreath, you will find yourself zeroing in on (II.11), "A chaplet or garland worn as a mark of distinction or honor."
Still, you have at this point to get to wreathed as an adjective, which is how Prufrock is using the word. But if you let def. II.11 of wreath guide you, you'll see definition 5 leaping off the page: "Covered or decked with a wreath, garlanded."
Still, you have to make absolutely sure. After all, people might be decked with a wreath for reasons having nothing to do with the ancient Greek tradition of decking a victor with a laurel wreath. (The Hawaiian hotels that drape you in leis when you arrive, for instance.)
So how do we know that Prufrock's wreath means something like "I have imagined myself decked with a wreath or garland, as winners of contests in ancient Greece were"? This is where remembering that England's tradition of the "poet laureate" descends directly from the Greek custom of crowning victors with laurel becomes relevant.
Under def. 5, the OED shows the poet Byron -- an accomplished satiric poet who was merciless about putting down what he considered bad poetry -- making fun of a contemporary poet (Robert Southey) whom he didn't think deserved to be the Poet Laureate of Britain. Byron specifically mocks
So now the OED has takes us to the primary meaning of Prufrock's "by sea-girls wreathed with seaweed red and brown." But there did the seaweed come from, you may ask? The answer is that the imaginary sea-girls who crowned Prufrock in those earlier moments when he was still able to imagine himself victorious don't have access to laurel, the sacred plant used by the Greeks to symbolize victory. It grows on land. So the mermaids use what they have on hand, making their wreath of seaweed, red and brown. And there we have the explanation of the image in all its sad (for Prufrock, and for us if we sympathize with him) specificity.