不期而遇 [- - ér yù]meet sb. accidentally [unexpectedly; by chance]; chance on [upon]; come [run] across; encounter; fall across; happen to meet; meet without expecting to; meet without previous arrangement; meet with sb. by chance:
•¶ 我在图书馆与一位老朋友不期而遇。 I ran across an old friend of mine in the library.