The crossword solution for the term SPARE NOT THE ROD is SPANK, which is a five-letter word. This phrase is an idiom that means to punish someone severely, usually with physical punishment such as spanking. It is derived from the Bible, where it is written in Proverbs 13:24, 'He who spares the rod hates his son, but he who loves him is careful to discipline him.' This phrase is often used to emphasize the importance of discipline and the consequences of not disciplining someone.