end, exhaust
竭 centers on using something up completely: bringing something to its end or exhausting a resource until nothing remains.
竭 combines 立 (stand) with 曷, which likely contributes the sound. The exact historical development of the meaning 'exhaust' from these components is uncertain.
A person 立 standing beside a complex shape 曷 that looks like a container being emptied: standing there until everything is exhausted and comes to an end.
For ケツ, imagine a kettle (ケツ) boiling dry until all the water is exhausted: kettle -> ケツ, and the kanji means to use up completely.
the flame that burns twice as bright burns half as long; the well with sweet water will be the first to run dry