consuetudo

Latin consuetudo means 'habit, usage, practice,' from consuescere ('to become accustomed'), from con- ('together') and suescere ('to become used to'). Roman law recognized consuetudo as a source of legal authority — established practice could create binding law. If people had always done something, that was reason enough to require it. Custom was law's older, quieter sibling.

Old French costume (later coustume) split the Latin word into two English borrowings. 'Custom' (the practice or habit) entered English in the 1100s. 'Costume' (the way of dressing) entered later, in the 1700s. Both trace to the same Latin root — your costume is what you have become accustomed to wearing. The split happened in French and was inherited by English.

The tax meaning — 'customs duties' — dates to the medieval period. Traders entering a port paid customary fees (consuetudinaria) to local authorities. The fees were 'customary' because they had been charged for so long that they had become tradition. The word for habit became the word for tax. English customs officers still work at borders, collecting what traders have become accustomed to paying.

The phrase 'custom-made' inverts the original meaning. A custom is supposed to be communal — what everyone does. A custom-made product is unique — what nobody else has. The word that meant 'the way we've always done it' now also means 'the way you specifically want it.' Communal habit and individual preference share a name.