Sageulse looks like a pure native Korean word, but it actually tumbled down from Chinese characters! Originally it was sagwolse (朔月貰). The character sak (朔), meaning the first day of the lunar month, also served as a unit for counting months, so sagwol meant "month by month," that is, monthly rent. Sagwolse, then, was simply "rent paid each month." Over time the diphthong in the second syllable loosened into a simple vowel, and the sound drifted: sagwolse → sageulse → sageulse. The fun part is the language policy. The form sagwolse was used into the 1950s, but acknowledging that the worn-down pronunciation had already set in, the 1988 standard-language revision adopted sageulse outright and discarded sagwolse. It is a rare case in which, even for a Sino-Korean word with a perfectly clear etymology, the changed sound was officially chosen over the original spelling.