It's a handwritten abbreviation for 開. 門 and 間 can also be abbreviated in the same way. They're some of the most common of a class of simplified variants called ryakuji (第 is another frequently abbreviated character).
Most ryakuji don't exist in Unicode or common fonts, so they aren't easily reproduced in electronic text. As a result, familiarity with them in online communities tends to be low.
It's a handwritten abbreviation for 開. 門 and 間 can also be abbreviated in the same way. They're some of the most common of a class of simplified variants called ryakuji (第 is another frequently abbreviated character).
Most ryakuji don't exist in Unicode or common fonts, so they aren't easily reproduced in electronic text. As a result, familiarity with them in online communities tends to be low.
What a great way to filter, bully or humiliate people online. TIL. Thanks to both of you and to Salome of course.