(adj) relating to small (not capitalized) letters that were kept in the lower half of a compositor's type case, Ant. uppercase, Example:lowercase letters; a and b and c etc
The Collaborative International Dictionary of English (GCIDE) v.0.53 Collaborative International Dictionary (GCIDE)
a. (Print.) Pertaining to, or kept in, the lower case; -- used to denote the small letters, in distinction from capitals and small capitals; as, lowercase letters a and b and c etc.. Contrasted with uppercase. See the Note under 1st Case, n., 3. [ 1913 Webster ]