Pertinacity

1846, Edgar Allen Poe, The Black Cat With my aversion to this cat, however, its partiality for myself seemed to increase. It followed my footsteps with a pertinacity which it would be difficult to make the reader comprehend.

Again and again, however, and half a dozen other agains, with the inexorable pertinacity of a child intent upon some object important to itself, did he renew his efforts for admittance.