Made for one case rather than for all. Also a practice repeated until it became expected — both senses are about what a particular group does.
Old French costume (habit, custom), from Latin consuetudo (habit, usage), from consuescere (to become accustomed), from com- (thoroughly) + suescere (to become used to); made for one particular case, and separately, a practice repeated until a group came to expect it.
Both senses share consuescere, becoming used to something, and the word never really splits: a custom practice and a custom-made thing are each shaped by one group's particular becoming-used-to. Neither sense claims to hold for anyone outside that group.