trout (n.)

Old English truht "trout," in part from Old French truite, both from Late Latin tructa, perhaps from Greek troktes "a kind of sea fish," literally "nibbler," from trogein "to gnaw," from PIE *tro-, from root *tere- (1) "to rub, turn." In late 17c. slang, trusty trout was used in a sense of "confidential friend."