Twunk is a not common slang term combining twink and hunk. Think is typically a sexual term for gay/homosexual men. This is talking about a small looking guy that typically is the one to receive/"bottom" during sex.
"Look at that little twink!" It is not an offensive term, but you don't often hear it in public... As its typically a term used erotically.
A hunk is a retro/older term that people use to describe an attractive man.
"Wow, what a hunk!"
As of this year I believe, it was combined together to create Twunk as a joke. Since these two contradict each other, its often silly to hear someone say it.