The word 失效 is formed by directly combining two meaningful characters to create a new, logical concept. The first character, 失 (shī), conveys the idea of "losing" or "failing". The second character, 效 (xiào), represents "effect", "efficacy", or "function". When placed together, they sequentially describe a process or state: the loss of effect or the failure to function, perfectly capturing the meaning of "to become invalid" or "to expire". It is a straightforward conceptual pairing where the first character acts upon the meaning of the second.