A while back, I got curious about how long the term mesugaki had been around, and fortunately, someone had already looked into it. Most of this info comes from that page and from the Japanese Wikipedia page for mesugaki.

The term (not in the present sense) has been used since at least as early as 1982, when it appeared in chapter 137 of Urusei Yatsura, 「口づけと共に契らん！！」 ("Sealed with a Kiss!!") (volume 14, page 33). It also appeared in chapter 6 of Black Lagoon in 2002.

It probably started being used as internet slang (still not with the present meaning) due to Inmu and Cookie☆, as a derivative of the insult "homogaki" for kids watching Inmu videos (and bothering others). This usage dates back to at least 2013. As an aside, the artist Haguran started calling their OC Mesugaki in 2013, and that character had an entry made on niconicopedia in 2014. But comments from that year on niconicopedia say that for disambiguation, the general meaning ("insult?") should also be mentioned.

The present meaning seems to date back to August 2018, after the announcement of Grim Aloe being added to Bombergirl—that announcement being the source of this post. The word appears a few times in replies within hours of the original announcement, and started appearing on images of Grim Aloe on pixiv soon after.

Pool context:

Laethiel said:

The present meaning (of the term mesugaki) seems to date back to August 2018, after the announcement of Grim Aloe being added to Bombergirl—that announcement being the source of this post. The word appears a few times in replies within hours of the original announcement, and started appearing on images of Grim Aloe on pixiv soon after.