Plain English Meaning
FU on a weather report means there is smoke in the air at the airport, and that smoke is reducing how far pilots can see.
Full Definition
In a METAR weather report, FU is the contraction used to indicate smoke as an obscuration to visibility. It denotes suspended combustion particles in the air — typically from wildfires, agricultural burning, or industrial sources — that reduce visibility at or near the reporting station.
Context Anchor
Seen in the weather portion of a METAR when smoke is affecting the airport area.
Derivation
The contraction FU comes from the French word 'fumée,' meaning smoke. Many METAR weather codes are based on French because the format was standardized internationally, and French was a working language of early aviation . Knowing this helps because FU has no obvious to the English word 'smoke' — the only makes sense once you see the French .