Mushroom Cloud

It depicts a blast plume, strongly associated with nuclear weapons.

Smoke, dust, and debris rise in a stem and spread into a cap. The nuclear image's public history began at Trinity, the first atomic bomb test, on July 16, 1945. A documented computer-era source for this exact glyph is the Amstrad CPC home computer: it stored the picture at character slot FD, a machine-specific code rather than a modern Unicode typing shortcut.

The Hiroshima Peace Memorial Museum archives eyewitness drawings of the cloud above Hiroshima on August 6, 1945. By 1946, a reporter at the U.S. Bikini Atoll tests called it an Atomic Age emblem. U.S. advertising and entertainment could sell atomic power as glamorous spectacle, while a protest image from the period turned the same shape into a warning of death and misused power.

What does it mean if someone sends this symbol?

Read it first as a reference to nuclear disaster or its threat.

The image cannot tell you whether the sender means history, an actual threat, or dark humor. Beside a report of real violence or a memorial, it can read as flippant; let the surrounding conversation decide.

Does a mushroom cloud always mean a nuclear blast?

No; other events can produce a similar plume.

Very large non-nuclear explosions and volcanic eruptions can raise a comparable stem-and-cap cloud. Seeing this silhouette alone does not identify what caused it.