To do something that inadvertently leads to trouble. The phrase refers to Cadmus, a figure in Greek mythology who killed a dragon and, on instructions from Athena, sowed (planted) its teeth. The soldiers that grew from the teeth fought each other until only a few remained. A: "Why did you invite Joe, my sworn enemy, to this party?" B: "Listen, if I had known that I would be sowing dragon's teeth by inviting Joe, I never would have done it!" I guess I sowed dragon's teeth when I complained about my neighbors being too loud because now they won't talk to me.