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1.
Impressive at first but with experience, found to be stupid or unimaginative
"That John Kerry seems to have gravitas."
"He's a mile wide and an inch deep."
by octopod April 10, 2004
 
2.
said of a person's knowledge, intelligence or, in some cases, opinion. put another way, you're saying the person knows a lot about everything, but nothing about a given topic.
"That Sarah Palin sure knows foreign policy!"
"She's all talking points. Her knowledge is a mile wide and an inch deep."
by OuLiPo February 27, 2010