flint n.

Old English flint "flint; a type of rock noted for hardness and for giving off sparks when struck," from Proto-Germanic *flintaz (source also of Middle Dutch vlint, Old High German flins, Danish flint), from PIE *splind- "to split, cleave," from root *(s)plei- "to splice, split" (source also of Greek plinthos "brick, tile," Old Irish slind "brick"), perhaps a variant of *spel- (1) "to split, break off." Transferred senses (hardness, etc.) were in Old English.