third cousin
Refers to a third cousin: the child of one's parent's second cousin, or the grandchild of a great-grandparent's sibling.
He is my third cousin.
We rarely have a chance to get together as third cousins.
いとこ means 'cousin' (first cousin). みいとこ is a more distant relative, specifically a third cousin.
はとこ means 'second cousin'. みいとこ is one generation further removed.
Compound of 三 (mi, 'three') and 従兄弟 (itoko, 'cousin'), literally 'third cousin'. The reading みいとこ is a contraction of み (mi) and いとこ (itoko).