Fish-Eating Grin Meaning

(idiomatic) A broad smile indicating self-awareness that may suggest self-satisfaction, smugness, discomfort, or embarrassment.

Example: 2002, Brian Cruver, Anatomy of greed: The Unshredded Truth from an Enron Insider, ISBN 9780786710935, p. 337:
  I had a few questions for him myself, but I just stood there and stared. He had a fish-eating grin on his face.
2008, Joel Stratte-McClure, The Idiot and the Odyssey: Walking the Mediterranean, ISBN 9780977586653, [1]:
  Then the Gypsy makes a crack about my accent. "You talk like a Spanish cow," he says with a fish-eating grin.