Assailants have forcibly tattooed a word on a young man's forehead during a bizarre and sustained assault at a Wollongong home.

The 18-year-old was also assaulted with a baseball bat over several hours, causing serious injuries.

The attack took place when the man went to a home in Albion Park Rail, south of Wollongong, on February 23, police said.

Police will not say what was tattooed on his forehead.

"He was also kicked and punched in a sustained assault," police said in a statement.