The term 'stack up' can be defined by four crossword solutions: RANK (4 letters), PILE (4 letters), AMASS (5 letters), and RATE (4 letters). RANK is a verb meaning to arrange people or things according to their relative importance or quality. PILE is a noun meaning a large, disorganized collection of objects. AMASS is a verb meaning to accumulate or gather over time. Finally, RATE is a verb meaning to assign a numerical value to something based on its quality or quantity. Together, these words suggest that the phrase 'stack up' means to organize, accumulate, and evaluate something.