A Korean slang / meme for 'Hello'. 'Bo-' is a prefix for Bokyem, a Korean famous youtuber, and '-iru' means Hairu which is an old-fashioned expression to say
hi in South Korea. It was at first just how Bokyem say
hi on his intro, but spread through the
Internet as Bokyem became famous, and even people who are not a Bokyem's fan started to use this word.