You have a tall sow thistle (Sonchus) in your yard.
I agree with Bamboo that it isn't a particular healthy specimen, as it shows a lot of powdery mildew (the white dusty areas) but unless you plan on nursing what is considered a weed in Europe and an invasive plant in the US, you probably are going to rip / dig it out anyway.
Doing some further research, you might have a Fireweed (Erechtites hieraciifolius or Sonchus agrestis) which, as the latin names show, sometimes is classified as of the genus sonchus, sometimes as erichites. It's native to the US and is a pioneer plant on burn areas and similar. (Apart from possibly being native instead of invasive, the previous answer remains valid.)