Well, you have us a little stumped, but we have come up with a couple of likely options for this mystery plant. Identity of a plant is better established if the flowers can be seen, growth habit can be observed and the plant origins known (self seed or nursery grown, planted & forgotten or escapee from a neighbouring garden underneath the fence). Not knowing any of this information makes a definitive ID difficult.
The options we have come up with are either, Purple Amaranth or Purple Basil.
Below are several links to information on Purple Amaranth.
Crushing a leaf of the plant in your hands and smelling it will lead you closer to determining if it is a purple form of Basil. That is, if you are familiar with what Basil smells like. Here too, is a link to information on this plant.
Now that you have the above information, I would recommend that you do not ingest any of either of these plants until you are 100% certain of their identity – better to be safe than sorry.