butt in seat. Slang. A term used to distinguish airline mileage accrued by actually flying, as opposed to miles awarded for using a credit card or some other method.
In context . . .
"... achieving elite status annually is becoming more and more elusive. Attaining Million Miler status with an airline is a way to lock in elite status. Each airline handles the milestone a little differently but commonly, it all begins when you have flown “butt in seat” as the saying goes, a million miles with a particular airline."
Source: frugaltravelguy.com
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