I’ve been spending too much time in the garden: my mind keeps trying to insist this word is said as mulch. No, the c is hard, like the effect on the person being mulcted.
It derives from classical Latin, in which multa or mulcta meant a fine or penalty. The c was probably introduced as the result of confusion with the verb mulcare, to handle roughly or damage, an unsurprising association of ideas. But many centuries passed before the change became general. Both Anglo-Norman and Middle French had multer, to pay a fine. It came into in English in the fifteenth century as mult and it was a century before the c became wedged permanently into place. (The same thing happened in French, in which mulcter evolved from multer at about the same time, though the verb has disappeared from the modern language.)
Mulct remains in English, though in its original sense it is now restricted to the world of lawyers and judges. Elsewhere, it has shifted to refer to the illegal extraction of money through fraud or extortion. Since it appears in phrases such as “mulcting the poor taxpayer”, we may assume people subject to some legally sanctioned mulct came to regard it as excessive.
The [energy supply] industry is quick to pass on price increases, and damnably slow to give customers the benefit of today’s tumbling costs. It is no good urging these hardfaced profiteers. The only thing they understand is the brute force they use to mulct their customers.
Daily Mirror, 12 Dec. 2008.
Scots once had a related term, multure, based on the older spelling of mulct, which can be traced back to the early thirteenth century for a charge or toll made by a miller, a multurer, for grinding corn. Knowing something of the rapaciousness of old-time millers, the association must have helped the shift in sense of mulct.