Lowermost means lowest of all. The lowermost branch, the lowermost shelf.
It is a superlative built the same way as uppermost and innermost, and English makes very few of them now. It is formal and belongs to description rather than to instructions.
1adjectivelowermostLowest of all.
“The lowermost branches had been cut back to head height.”
lowermost is a position.
From lower + -most.
Etymology from Wiktionary, CC BY-SA