If it has a waxy underside it should be a lambsquarter. I like it as a spinach substitute. A well cared for plant will grow really tall, I have some that are 8 feet tall. To harvest just go find where the stem starts to harden, go back a little to where it is tender, and break off this will be the point at which new growth spreads.

Maybe a species of Chenopodium genus (goosefoots aka lambquatters). I’m guessing it might be Chenopodium album? Web site below also has a Google link to images of C. album. Check it out and see if you have a match. Best wishes! http://www.cbif.gc.ca/eng/species-bank/canadian-poisonous-plants-information-system/all-plants-scientific-name/chenopodium-album/?id=1370403266807