Completely full, especially to the point of overflowing; filled to capacity with something.
From Middle English 'brim' (edge, border) of uncertain origin, possibly related to Old Norse 'barmr' (rim). The sense of 'full to the brim' developed in the 16th century, with the participial adjective following.
The image of liquid reaching the very edge of a container perfectly captures fullness and abundance. Interestingly, 'brim' originally just meant 'edge' (as in hat brim), but through the metaphor of containers, it became synonymous with maximum capacity and enthusiasm.
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