Old English suðerne, from suð "south" (see south) + -erne, suffix denoting direction. A common Germanic compound (Old Frisian suthern, Old Norse suðroenn, Old High German sundroni). The constellation Southern Cross so called in English by 1756.
superlative suffix of adjectives and adverbs, Middle English alteration (by influence of unrelated most) of Old English -mest, a double superlative, from -mo, -ma (cognate with Latin -mus; compare Old English forma "first," meduma "midmost") + superlative ending -est. Now generally mistaken as a suffixal form of most.
<a href="https://www.etymonline.com/word/southernmost">Etymology of southernmost by etymonline</a>
Harper, D. (n.d.). Etymology of southernmost. Online Etymology Dictionary. Retrieved $(datetime), from https://www.etymonline.com/word/southernmost