Wiktionary
vb. (context baseball English) Of a baserunner, when a fly ball is hit, to put one's foot on the base one is currently at until the ball is caught. When the ball is caught, the baserunner may attempt to advance to the next base, at the risk of being tag out.
Wikipedia
In baseball, to tag up is for a baserunner to retouch or remain on their starting base (the time-of-pitch base) until (after) the ball either lands in fair territory or is first touched by a fielder. By rule, baserunners must tag up when a fly ball is caught in flight by a fielder. After a legal tag up, runners are free to attempt to advance, even if the ball was caught in foul territory. On long fly ball outs, runners can often gain a base; when a runner scores by these means, this is called a sacrifice fly. On short fly balls, runners seldom attempt to advance after tagging up, due to the high risk of being thrown out.
When a baserunner fails to tag up on a caught fly ball (for instance, if they started running too early, thinking the ball wouldn't be caught), they may be "doubled off", which results in them being called out. To double a runner off, a fielder must touch the runner's starting base while in possession of the ball, before the runner returns to the base. If the baserunner appeared to tag up, but a fielder suspects the baserunner may have left the base too early (thus failing to legally tag up), the fielder may attempt to double the runner off by touching the runner's starting base while controlling the ball, before the next pitch is thrown. This is considered a type of appeal play. If the umpire agrees that the runner did not retouch after the ball was touched by a fielder, the umpire will call the runner out, and anything else the runner did during the play (such as score a run) is negated. Doubling a runner off is considered a "time play" (as opposed to a force play), which means that even if the doubling-off is the third out of an inning, any runs which score before the double-off will count (unless the run was scored by the same runner that was doubled off, in which case the run will not count in any situation).
Usage examples of "tag up".
Scott picked Tag up in his arms and promised to keep him there, the attendant finally consented that he should do so.
He paused at the first red fire bottle he came to and flipped the inspection tag up so he could read it.
Setting down the sidearm, he took up the chain, activated the desk's illuminant with a motion, and held one tag up to its light.
Denyabe would tag up on the play and tie the game, keeping the Icebreakers’.
He watched the swarms of children playing tag up the mountainsides to the berry patches.
Major Tannis Gateau, Imperial Cadre Medical Branch, turned her name tag up to scrutinize it, then nodded.
Building one with the new launchers pushed its price tag up to about a quarter of a destroyer’.
Building one with the new launchers pushed its price tag up to about a quarter of a destroyer's, especially with the fire control upgrades to take full advantage of the launchers' capabilities, yet Hemphill had lobbied hard for the resumption of LAC construction, and she'd succeeded.
Major Tannis Cateau, Imperial Cadre Medical Branch, turned her name tag up to scrutinize it, then nodded.
They let it lay around a while, and then if it can't be reworked and sent back to Final, they scrap it and send the rejection tag up to the office.
Building one with the new launchers pushed its price tag up to about a quarter of a destroyer's, especially with the fire control upgrade needed to take full advantage of the launchers' capabilities, yet Hemphill had lobbied hard for the resumption of LAC construction, and she'd succeeded.
Turkle got out flashlights, and we played tag up and down the hall with wheelchairs from storage, having a big time till we heard one of Sefelt’.