Lewis Short
mātrŭēlis (noun M) : mater
* A mother's brother's son, a cousin-german, first-cousin on the mother's side (cf. patruelis, a father's brother's son; post class.),Dig. 48, 9, 1; Vict. de Orig. Gentis Rom. 13, 8.
Charlton T. Lewis, Charles Short, A Latin Dictionary