Meaning
The letters AB together in a square, usually red, the label for blood type AB. It is one combined symbol rather than 🅰️ plus 🅱️.
It mostly appears in blood donation posts, medical forms and blood-type personality jokes, a pastime many fans of Japanese and Korean culture enjoy: “Turns out I’m 🆎, the odd one out in the family” It can also abbreviate anything shortened to AB, like a name or initials.