Trenched on is a phrase that is used to describe a situation where something has been encroached upon or invaded. The idea of trenched on can be synonymous with words such as interfered, encroached, violated, or trespassed. Other synonyms for this phrase could include the words impinged, infringed, intruded, or intruded upon. Essentially, the phrase trenched on implies that there has been some sort of boundary that has been crossed, whether that be physical, emotional, or social. In any case, using synonyms for this phrase can help to provide more nuanced descriptions of the specific context in which it is used.