Keira Knightly has prominent bone structure with high defined cheekbones and a strong square jawline. She also has "wonky teeth", not an underbite.
Celebrities with Crooked Teeth via kwout
You might be thinking of Joe Mande's curated GIFs of Knightly, who plays a "Crazy Person", taken from "A Dangerous Method". See Keira Knightley’s Underbite Acting at BuzzFeed.
Underbite is a condition in which the lower front teeth overlap and partially cover the upper front teeth. In many cases, underbite has a skeletal component. For example, it may be caused by mandibular prognathism, or the protrusion of the lower jaw such that the lower teeth do not align with the upper teeth. In other cases, underbite is simply a dental condition, and may be caused by childhood behaviors like thumb-sucking or the persistent thrusting of the tongue against the teeth.
Check out 10 Celebs Who Refuse To Fix Their Teeth featured at The Huffington Post.