I’m rereading Moby Dick after a lapse of decades, and in the famous chapter “The Whiteness of the Whale” I was stopped by this passage:
As for the white shark, the white gliding ghostliness of repose in that creature, when beheld in his ordinary moods, strangely tallies with the same quality in the Polar quadruped. This peculiarity is most vividly hit by the French in the name they bestow upon that fish. The Romish mass for the dead begins with ‘Requiem eternam’ (eternal rest), whence Requiem denominating the mass itself, and any other funereal music. Now, in allusion to the white, silent stillness of death in this shark, and the mild deadliness of his habits, the French call him Requin.
Needless to say, I found the suggested etymology more than dubious, and went to investigate. Imagine my surprise when I discovered the following entry (updated March 2010) in the OED:
Etymology: A borrowing from French. Etymons: French requin, requien, requiem.
< French requin (1539 in Middle French), requien (1578), requiem (1658 in the passage translated in quot. 1666), of uncertain origin; it has been suggested that the second element may show French regional (northern) quin, variant of chien dog (compare chien de mer dogfish n.), but this explanation poses difficulties. The form requiem probably results from folk-etymological association with requiem requiem n.1, a person taken by the shark being taken to be as good as dead. Compare Portuguese requeime (of uncertain date and origin).
A large or dangerous shark; (Zool.) (more fully requiem shark) a member of the family Carcharhinidae, which includes many of the typical large and medium-sized sharks (as the tiger, bull, blue, and reef sharks) and the hammerheads.
1666 J. Davies tr. C. de Rochefort Hist. Caribby-Islands i. xvii. 102 The Requiem, otherwise called the Shark-Fish, is a kind of Sea-Dog or Sea-Wolf.
1666 J. Davies tr. C. de Rochefort Hist. Caribby-Islands i. xvii. 103 The French and Portuguez commonly call it Requiem, that is to say Rest, haply, because he is wont to appear in fair weather.
1705 tr. W. Bosman New Descr. Coast of Guinea xv. 281 Hayes or Requiens, by some (though utterly wrong) named Sea-Dogs;..are very thick as well as very long, some of them betwixt twenty and thirty foot.
1896 D. S. Jordan & B. W. Evermann Fishes N. & Middle Amer. I. 27 [Family] Galeidae. (The Requiem Sharks)… Sharks with 2 dorsal fins, the first short and high, entirely before the ventrals.
1973 ‘P. Buchanan’ Requiem of Sharks xiii. 136 Any man-eater is called a requiem.
2000 C. Tudge Variety of Life ii. xiv. 362 The Carcharinidae [sic] (13 genera, 15 species) contain the somewhat chillingly but aptly termed ‘requiem sharks’; they range from medium-sized to extremely large.
So the etymology is indeed dubious, but what a great word!