To consider something carefully and thoroughly before making a decision.
This phrase uses 'over' to suggest repetitive or comprehensive mental action, like turning something over in your hands to examine all sides. The construction became common in English by the 17th century, emphasizing deliberate rather than hasty thinking.
The 'over' in this phrase captures the recursive nature of serious thinking - we don't just think once and decide, we circle back, reconsider, and examine our thoughts from different angles, much like a jeweler examining a stone.
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