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1.
A small basketball-sized alien found in the assumed fictional game Half-Life.

Headcrabs are small light yellow domes with four legs. Their two front legs are long grasping claws, and the two rear are simply stubs to walk on and jump with. Headcrabs find a human-like host and leap for the head, hence the name. They orient themselves on the head and cover the host's face. Their mouthlike structures somehow take control of their host's brain activities and keep them alive as the headcrab. The "zombie" then conforms over times to include long pointy claws on the hands and a mouth that opens between the ribs.

A fully grown headcrab turns into a gonarch, commonly called Big Momma. Four long armored legs and a circular shell to cover the top, gonarch is radial in symmetry. She has a large eggsack and gives birth to baby headcrabs often. Few headcrabs live to this age.
The quick foxy bullsquid jumped over the lazy headcrab's host.
by Tiki Man January 10, 2004
 
2.
Headcrabbing is the term for when an individual sneezes or nearly sneezes semen out of their nose. This action is typically by incident, but can be purposefully done if the nose is tickled while you are swallowing semen.
Hey Amelia, did you hear about that girl Ama, her boyfriend said she headcrabbed last night.
by Mallowin June 21, 2013
 
3.
See headcrab.
The quick foxy bullsquid jumped over the lazy headcrab's host.
by Tiki Man September 09, 2003
 
4.
a comical way of saying head lice. the opposite of (but still very similar to) pubic lice.
Hey, thanks for letting me borrow your hat. There's only a small problem, though:
you gave me head crabs, ass-fuck!
by Cilantro November 02, 2008
 
5.
Headcrabs are from Half-Life, but this is a Half-Life 2 Headcrab. Headcrabs are 4 leged creatures that hop on Citizen's head and turn them into a Zombie. Headcrabs had some upgrades, they can barry themselfs in sand or dirt and pop out and hop on your head (really for you they just scratch), the Combine had taken Headcrabs from Xen and sent them on rockets and launch them in non-combine areas such as the small town of Ravenholm.
To destroy a Headcrab, just use your crowbar, it's the easiest way. If you don't have the crowbar when you fight a Headcrab (like in HL2: Episode One), use a pistol, SMG, or shotgun. Headcrabs can be tricky to kill, also 3 shots from your Gravity Gun can kill one in Half-Life 2.
by Mega Sean 45 June 18, 2008
 
6.
lice, bugs that live on your head
yo, don't go near that kid,he got head crabs.
by check_minus May 17, 2008