To travel at a speed exceeding 100mph and/or well above normal limits. A buck is 100 cents (or $1.00), hence for 100mph being a buck being the origin of this term.
That car was bucking it down the highway. The woman with the green shorts was bucking it past the other runners. That boat was inappropriately bucking it in a no-wake zone.
12 step recovery jargon referring to someone new who talks about how great life is, now that they'resober. Usually meaning that the person is out of touch with reality.
The new guy seems pretty happy for a dude who has no job, no money and no family. He must be on a pink cloud.