To make an unnecessary fuss, moan, bother.
The European Commission is pondering how to compel phone companies to come up with a universal plug for their chargers. Apple, which uses its own design for its charger cables, is mithering about the change. ("The parable of the plug", The Economist, 6 February 2020)
To pester or irritate someone. Usually directed at children.
Will you stop mithering me!
Mother