Male Sign

It labels a person, animal or group as male. You meet it on forms and in science, where ♂ and ♀ stand for male and female (a litter listed as "2♂ 3♀", a hamster tagged ♂ in a pet shop). People put it in dating and social profiles next to their age, sports fixtures use it for the men's draw, and family-tree and medical charts use it as shorthand so a diagram stays readable.

It is also the planet Mars. Astronomers used the sign for centuries, and astrology still does: a post about "Mars in Aries ♂️" is about the planet, not about men. The two meanings are one history. The Roman god Mars was male and warlike, Venus female, so their planet signs were borrowed for the two sexes.

Why is ♂️ sitting after another emoji?

Because that emoji was a man, and your device split him in two. Emoji such as 🏃‍♂️ (man running), 👮‍♂️ (man police officer) and 🤦‍♂️ (man facepalming) are not single pictures. Behind the scenes they are a person emoji, an invisible joining character and ♂️. A phone or app that knows the sequence draws one figure; one that doesn't shows the figure and then a loose ♂️. The sender typed one emoji and meant one thing.

Sent on its own, ♂️ is a plain label, not a mood. Someone replying "♂️" to "boy or girl?" means "a boy"; a pet listing "♂️, 3 months" means a male puppy. Paired with ♀️ it means both sexes or a man-and-woman couple, and doubled, ♂️♂️, it is the long-standing sign for gay men. Tagging another person's gender with it uninvited can read as a jab in arguments about identity, so use it on yourself and on your cat.

Is it really a shield and a spear?

Probably not, however good the story is. The popular explanation says the circle is the war god's shield and the arrow his spear. Historians trace the shape instead to medieval and Greek scribes contracting the written name of the planet into a few strokes, as they did for other planets too.

The sign entered biology in 1751, when Carl Linnaeus used ♂ and ♀ in his notes on plant breeding to mark male and female parents. Botanists kept it, zoologists followed, and from there it reached medicine, pet shops and eventually keyboards. Alchemists had already used the same mark for iron, the metal of weapons, which is why Mars, iron and maleness share one symbol.