The word 思议 is formed by combining the character 思, meaning "to think", with 议, meaning "to discuss" or "deliberate"; this combination results in a word meaning "to conceive" or "fathom", expressing the mental action of considering and reasoning through ideas.
Word Definition - 思议
sī yìto imagine; to comprehend (usu. used in the negative, as in 不可思議|不可思议[bùkě-sīyì] "inconceivable")