The amount or number of people that a carriage can hold at one time.
Carriage + full (the amount needed to fill something). This Old English-derived suffix -ful creates nouns meaning 'the quantity held by' a container.
A 'carriageful' was a standard unit of measurement for capacity in the 1700s-1800s—if you said 'a carriageful of visitors arrived,' everyone understood immediately how crowded your drawing room would be.
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