Heap up is a term that can be used to describe the act of gathering or accumulating a large amount of something. The crossword solutions for this term are AMASS (5 letters), CUMULATE (8 letters), PILE (4 letters), and COMPILE (7 letters). All of these words have similar meanings, with AMASS and CUMULATE referring to the act of gathering a large quantity of something, while PILE and COMPILE refer to the act of arranging or organizing a large quantity of something.