1. superlative form of mum; most silent, quiet, or reluctant to speak
Usage: informal; chiefly British
2. superlative form of mum; most characteristic of or resembling a mother; most motherly
Usage: informal; chiefly British

Examples

• He remained the mumsiest of all the witnesses, refusing to answer any questions.
• She gave him the mumsiest look of concern when he came home sick.
• The mumsiest member of the group always kept everyone's secrets.
• His mumsiest response was simply a nod and a smile.
• She was the mumsiest about the whole situation, saying nothing at all.