A cirsoid aneurysm is the dilation of a group of blood vessels due to congenital malformations with AV (arterio venous) shunting.
Cirsoid means resembling a varix. Sometimes, a minor traumatic episode, such as a fall or bump on the head, can lead to the formation of a cirsoid aneurysm. Often these are trivial traumatic episodes
Cirsoid aneurysm in general is a hemangioma of an artery. It most commonly occurs over the head usually the superficial temporal artery and also it's branches. It can also occur in places where medium vessels lie over bones without much intervening tissues between them and the skin.
The superficial temporal artery is the most commonly involved artery.
References
Cirsoid aneurysm Wikipedia(Text) CC BY-SA