The Collaborative International Dictionary of English v.0.48:
Granduncle \Grand"un"cle\, n. [Cf. F. grand-oncle.] A father's or mother's uncle. [1913 Webster] grand unified theoryWordNet (r) 3.0 (2006):
granduncle n 1: an uncle of your father or mother [syn: great-uncle, granduncle]