A woman in a red tunic and bearskin hat, the dress uniform of the guards at Buckingham Palace.
Women do serve in these ceremonial roles, and the emoji fits posts about that, about British tradition, and about trips to London. In chat it also means standing firm: on duty, unmoved, and not laughing at anyone's jokes.
Facts
Woman guard ๐โโ๏ธ is an emoji in the people and body category, under person role. It became part of the emoji standard in Emoji 4.0, released in 2016. It is a sequence that joins ๐ guard and โ๏ธ female sign. Devices that do not know the sequence show those emoji side by side. It comes in five skin tones, from light to dark. On GitHub, Slack and other apps you can type it as :guardswoman: or :woman_guard:. People also look for it with words like โbuckinghamโ, โguardโ, โhelmetโ, or โlondonโ.