dog-ear

dog-ear \DAWG-eer\, verb:

1. To fold down the corner of a page in a book.

noun:
1. (In a book) a corner of a page folded over like a dog's ear, as by careless use, or to mark a place.
2. In architecture, another term for a crossette.

This was uncharacteristic of him, territorial as he was over books, always reminding me not to dog-ear pages.
-- Kaye Gibbons, Sights Unseen

I will dog-ear the pages, maybe even fill out the order form, but I won't get anything.
-- Elizabeth Berg, Until the Real Thing Comes Along

Dog-ear as a metaphor for the folded pages of a book first appears in the 1650s.