The term 'most rigid' can be described by two crossword solutions: 'tensest' (7 letters) and 'severest' (8 letters). Both words refer to something that is extremely strict, SEVERE, or inflexible. 'Tensest' is derived from the word 'TENSE, ' which means a state of being stretched tight, while 'severest' is derived from the word 'SEVERE, ' which means very harsh or strict. Together, these two words describe something that is extremely rigid and inflexible.