The brother or brother-in-law of one’s mother.(noun)
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Use "maternal uncle" in a sentence
"At a tender age he came to Rome to be educated in the monastery of Santa Maria on the Aventine Hill, over which his maternal uncle Laurentius presided as abbot."
"Shahriman showed the more kindness to Julnar and honoured her with increase of honour; and the little one grew up and flourished, whilst his maternal uncle and grandam and cousins visited the King every few days and abode with him a month or two months at a time."