The word "猝不及防" is formed by the sequential combination of four characters: "猝" meaning sudden or abrupt, "不" meaning not, "及" meaning to reach or in time, and "防" meaning to defend or guard against. These characters are arranged in a logical order where "猝" modifies the following phrase "不及防", which itself is a verb-object structure meaning "not in time to defend". The entire phrase literally translates to "sudden and not in time to defend", resulting in an idiom that conveys being caught off guard or unprepared for an unexpected event.