Meaning

noun the characters that were once kept in bottom half of a compositor's type case

adjective relating to small (not capitalized) letters that were kept in the lower half of a compositor's type case

originFrom the noun phrase lower case, which itself consists of an adjective modifying a noun; when used as a noun adjunct then, to render its syntactical function clearer and simpler, it is written as a single word or with a hyphen as a sort of chunking.

Etymology from Wiktionary, CC BY-SA 3.0