Mousecraft refers to the excessive use of the mouse in applications where the keyboard is infinitely more efficient.
A familiar example of a time you have probably practiced mousecraft, would be when you need to enter your date of birth on a website, and instead of simply typing it in the appropriate box and hitting "Enter" or "Return" you click on a dropdown calendar and spend several minutes clicking the back button to get from 2015 to 1958, and then another minute clicking through the months and hunting for the specific date, and then, once the date is entered in the box for you, spend a few more seconds looking for the affirmative button which might say "Ok", "Go", "Enter", "Continue" or something else, and then clicking on it. This is rank mousecraft.
Most habitual practitioners of mousecraft are hopelessly computer-illiterate, and probably can't touch-type, and probably insist on working with only one monitor when their employer offers them two. They are often but not always old, and old people are often but not always guilty of mousecraft. They are probably your mom.
"Why are you spending half an hour to enter with your mouse what you could type in two seconds? That sh*t is some serious effed-up mousecraft. No wonder you're so unproductive."