The present participle of gage: the act of measuring, assessing, or estimating the size, volume, or capacity of something.
From 'gage' (to measure) plus the '-ing' suffix. The '-ing' form creates a gerund or continuous form, allowing 'gage' to function as a noun (the gaging of containers) or a present action (they are gaging the liquid).
Interestingly, both 'gaging' and 'gauging' are used today, with 'gauging' becoming more common—but 'gaging' preserves the older spelling and connects directly to the historical 'gage' profession!
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