maun1 verb must la14-. etymology: Scots; northern Middle English man, Old Norse man; attested by SR, JS and BW
mun, maun noun the mouth, face or jowls of a person: His mither could hardly look at her laddie, and his faither would munt [weep] whin he deeked [looked] at his bonnie laddie's maun. 20-. Compare moi, moy, munt etymology: perhaps a diminutive from Scots munnvik; early Scandinavian; or a development from German mund ‘the mouth’; apparently attested earlier as a Middle English surname, as Munne (1287–8); also recorded earlier in place names; attested by Galloway and Perthshire and Argyleshire Tinkler-Gypsies and BS in TDITA; form maun attested by ET, SR and in Shelta; collected by EMcC/PS note:

The third sense of this in the Dictionaries of the Scots Language is defined as ‘an old person with a small shrunken face’ which may connect it to munt. Form mun attested by Canadian Paul Pope (2013) with the same meaning.