A container things are dropped into together. Bucketing groups by rough similarity rather than exact kind.
Anglo-Norman buquet (a pail), possibly related to Old English buc (a pitcher, a bulging vessel, also 'belly'); the exact line of descent is debated, but the image of a rounded, carried vessel runs through every proposed root.
Bucketing trades precision for speed — a rough bin is faster to sort into and cheaper to search than an exact match, which is why counts, rates, and time windows all reach for buckets before anything finer is worth the cost.