Tighten up is a phrase that is often used to describe the act of making something more secure, rigid, or FIRM. The crossword solutions for this term are TENSE (5 letters), STIFFEN (7 letters), and FIRM (4 letters). TENSE means to become or make TENSE, which can refer to becoming more rigid or strained. STIFFEN means to make or become stiff, which can refer to making something more rigid or secure. FIRM means to make or become FIRM, which can refer to making something more solid or secure. All three words can be used to describe the act of tightening up something.