In baseball, to return to one's current base and touch one's foot to it until a fly ball is caught by a fielder, after which the baserunner may attempt to run to the next base. He ran back and was barely able to tag up before the ball landed in the midfielder's mitt.
tag up
v.Baseball
To return to and touch a base with one foot before running to the next base after a fielder has caught a fly ball: The runner ran back, tagged up at first base, and then continued on to second.