The term 'clean by hard rubbing' can be summarized by two crossword solutions: 'SCRUB' and 'SCOUR'. Scrubbing is a cleaning technique that involves using a brush or other abrasive material to remove dirt, grime, and other debris from a surface. Scouring is a similar technique, but it involves using a more abrasive material, such as steel wool, to remove stubborn stains and dirt. Both techniques are often used in combination to achieve a thorough clean.