Capable of being filled; able to hold a substance or be completed, as in a 'fillable form' that can be written on or filled with information.
From 'fill' (Old English 'fyllan') plus '-able' (capable of), a straightforward modern compound describing something that can be filled.
PDF forms are called 'fillable' because you can type directly into them—but the word existed long before computers, describing anything from water bottles to tax forms that could accept something into them.
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