TENSE UP is a phrase that can be used to describe a situation in which someone or something becomes more rigid, alert, or anxious. The two crossword solutions for this phrase are TAUTEN (6 letters) and BRISTLE (7 letters). TAUTEN is a German word meaning 'to become tense' or 'to tighten, ' while BRISTLE is an English verb meaning 'to become stiff or erect, especially with fear or anger.' Both words can be used to describe the same phenomenon of becoming more rigid or alert due to tension.