Regular melee weapons that have rocket/jet engines attached to them for whatever obscure reason. Not limited to hammers despite the name.
Alternate names: Rocket-powered melee weapons
Name variations: Rocket hammers
The first video game about Jet hammers was released in 2004.
Also including any other technology that is claimed to accelerate the weapon to speeds not possible otherwise for the swings.
This type of item probably originates from anime/manga, with one of the earlier examples found in GunnM (Battle Angel Alita) and likely several even older mecha animes.