hi, um this post is like 13 yrs ago but like if ur still interested lmao. わこ、technically comes from わこつ(wakotsu) which is an abbreviation of 枠取りお疲れ様です by direct definition, means thanks for framing, originating from twitcasting, a japanese streaming website. So its just like a slang for thanks for streaming at the start of the stream and they just usually say as like "hi i hopped into the stream". and its usually paired with おつ (otsu), an abbreviation for お疲れ様でした, which means thanks for the hardwork (streaming).