The most disgusting, repulsive, or unpleasant; or the superlative form of 'gross' meaning the largest or most complete without deductions.
From Old French gros meaning 'large,' from Latin grossus. The superlative form adds -est. The sense of 'disgusting' developed from the idea of something being coarse, crude, or unrefined.
It's funny that 'gross' can mean both the most disgusting AND the total before deductions—English loves using the same word in totally different ways.
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