The son of one's nephew or niece; your grandnephew is two generations removed from you through your sibling's line.
From grand- (meaning 'one generation removed further') plus nephew (from Old French neveu, from Latin nepos meaning grandson or male relative). This term standardized in English during the 1800s as kinship terminology became more precise.
Many people don't actually use 'grandnephew' in everyday speech—they'll just say 'nephew's son'—but it exists to parallel how we say 'grandchild'!
Complete word intelligence in one call. Free tier — 50 lookups/day.