Fettle meaning : A state of proper physical condition; kilter or trim. One's mental state; spirits. Sand used to line a furnace. A person's mood or state, often assuming the worst. A seam line left by the meeting of mold pieces. The act of fettling. To sort out, to fix, to mend, to repair. To make preparations; to put things in order; to do trifling business. To line the hearth of a furnace with sand prior to pouring molten metal. To be upset or in a bad mood. In ceramics, to remove (as by sanding) the seam lines left by the meeting of two molds. To prepare.