The word 不假思索 is formed by combining the characters 不 (not), 假 (to borrow or rely on), 思 (to think), and 索 (to search), where 思索 together means "to think deeply". The sequence directly negates reliance on thought with 不 modifying 假, which then applies to 思索, resulting in the literal meaning of "not relying on thinking and searching" to convey acting without hesitation or spontaneous action.