1640s, from eastern + -most. Eastermost attested from 1610s; comparative eastermore from late 15c.
Entries linking to easternmost
eastern adj.
Old English easterne "of the east, from the east; oriental; of the Eastern Orthodox Church; of the eastern part of the globe," from east + -erne, suffix denoting direction. Cognate with Old Saxon ostroni, Old High German ostroni, Old Norse austroenn. Eastern Shore of Maryland and Virginia so called from 1620s.
-most
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.