The phrase "wert on tail" typically refers to someone who is following or chasing another person, often aggressively or with ill intent. There are several synonyms or alternate phrases that could be used in place of "wert on tail." For example, one might say that someone is "hot on the trail" of another person, or that they are "pursuing" or "hounding" them. Other possibilities include "dogging" or "shadowing" someone, or simply saying that they are "following closely." Regardless of the choice of words, the implication is usually that the person being pursued is in some sort of danger or jeopardy.