IPA: /ˈɡrændˌnɛf.ju/
KK: /grændˈnefju/
A child of one's niece or nephew.
My grandnephew is starting school this year.
Grandnephew is formed from "grand-" (meaning great or large, derived from Old French *grand*, meaning large or tall) and "nephew" (from Old French *neveu*, meaning a son of one's brother or sister). The term refers to the son of one's nephew or niece, thus indicating a generational step beyond a nephew.
Think of 'grand' meaning large or great, and 'nephew' referring to a brother's or sister's son. This helps you remember that a grandnephew is the son of your nephew.
No commonly confused words.