To ignore danger, unpleasantness, or the like by pretending not to see it. The term comes from the practice of ostriches, who spend a good deal of time burrowing headfirst in the sand. Most of the time they are eating, for these large, flightless birds consume sand and gravel, which are used in their gizzards to help digest food. However, it was long thought that they put their heads down in the mistaken belief that they then cannot be seen, and from the early seventeenth century on this mythical self-delusion was transferred to human beings who are avoiding unpleasantness.