When someone goes from one partner to another, then back to the first in rapid succession. It can happen any number of times, and with any number of people. Usually complicated to explain. Can also be related to girls, like a girlfriend tagger. Same concept applies.
"So Chris, are you with Nina, or Joanne?"
"Hmm...let's see... Joanne broke up with me, and got with Nick, and then broke up with him, got back with me, then went to Jonah, then back to Nick, and again with me, but I was sick of her boyfriend tagging, so I said it was enough, and got together with Nina."
"Oh. So she was a Boyfriend Tagger.
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