Lengthiest is an adjective that describes something as being the longest in terms of time, duration, or physical distance. There are several synonyms that can be used instead of lengthiest, such as longest, most extended, drawn-out, protracted, and interminable. These words are often used to describe situations where time seems to be dragging on, or where a physical object or entity seems to go on forever. For example, a boring lecture or long-winded speech could be described as the lengthiest or most extended one yet. By using synonyms for lengthiest, you can add more variety and nuance to your writing, making it more interesting and engaging.