Rearmost means furthest to the back. The rearmost carriage, the rearmost row of seats.
It goes before the noun and belongs to written notices more than to speech. Rearmost and hindmost mean the same, and the second is now nearly archaic.
1adjectiverearmostFurthest to the back.
“Only the rearmost carriage will open at this station.”
rearmost is a position.
Proto-Indo-European
*h₁rey- — to lift oneself, rise
Proto-Germanic
*raizijaną
Proto-West Germanic
*raiʀijan
Old English
rǣran — to raise, set upright, promote, exalt, begin, create, give rise to, excite, rouse, arouse, stir up
Middle English
reren — to raise
English
rear
English, today
rearmost
Built from
From rear + -most. rear comes from Middle English reren (“to raise”), from Old English rǣran (“to raise, set upright, promote, exalt, begin, create, give rise to, excite, rouse, arouse, stir up”), from Proto-West Germanic raiʀijan, from Proto-Germanic raizijaną, raisijaną (“to cause to rise, raise”), from Proto-Indo-European h₁rey- (“to lift oneself, rise”). Cognate with Scots rere (“to construct, build, rear”), Icelandic reisa (“to raise”), Gothic 𐍂𐌰𐌹𐍃𐌾𐌰𐌽 (“to cause to rise, lift up, establish”), German reisen (“to travel”) literally “to rear up and depart”; and a a doublet of raise. More at rise. Related to rise and raise, which is used for several of its now archaic or obsolete senses and for some of its senses that are currently more common in other dialects of English.
Etymology from Wiktionary, CC BY-SA