Manhacks are tiny machines that fly around and cut their targets with spinning blades. The blades they attack with are probably also the ones that allow them to stay in midair. Manhacks frequently open up special blades when charging at a target, possibly to attempt to latch into bodies.
In the Half-Life 2 beta, there was a manhack arcade where children would fly manhacks manually and attack people. Nobody seemed to question if the graphics were maybe a little too realistic...
npc_manhack, the entity representing a manhack ingame.