one checkbox of many should always stay unchecked

There are a lot of topics about javascript and conditional checking checkboxes, but I have not found one that matchs my case.

My usecase: I have generated a number of rows with checkboxes each row, lets say the number is n. Now the user should be able to check only n-1 of them. So in other words when we have 4 checkboxes and 3 of 4 are checked, the last one should be disabled. When only 2 of 4 are checked then all checkboxes are available.

I have tried:

var $checks = $('input:checkbox').click(function (e) {
        var numChecked = $checks.filter(':checked').length;
        var lastUnchecked = $checks.prop('checked', false);

        if (numChecked = $checks.length - 1) {
            lastUnchecked.disabled = true;
        }
        else {
            //enable checkbox that was disabled before
            lastUnchecked.disabled = false;
        }
    });

but this doesn't work because var lastUnchecked = $checks.prop('checked', false); return all checkboxes, not only unchecked