the.com/mum

means Chiefly British/Irish informal word for mother, and also an old adjective meaning silent — as in 'keep mum.'

from The mother sense comes from baby-babble — 'mama/mum' sounds being among the easiest for infants to make, a pattern echoed across unrelated languages. The silence sense is separate and older, from Middle English 'mum,' itself imitative of a closed-lips murmur or hum — basically the sound you make when your mouth is shut. 'Mum's the word' and keeping mum both trade on this second, unrelated 'mum.' The two meanings colliding in one word is coincidence, not kinship — language's little prank.