In French, ko threat translates to menace de ko. And muko to menace de con :-)
In this diagram, is muko.
 Robert Pauli:
Black's d can wait. He should instead close the gap above b, forming a two-point eye. Otherwise White walks in (twice) and Black loses six points. That in mind, White should not start at a but immediately walk in, costing Black four points compared to starting at a.