is it a threat if someone tells you that they are going to slam your head against a sink even if they are joking?

I've heard worse coming from my younger sister's lips and she's only 7. xD

Hmm... it depends, really. What kind of voice was the person saying the sentence in? If the voice tone was noticeably light-hearted, joking and warm - then it might've been just for laughs.

If the tone was deathly-serious, humourless and straightforward - then the person might have some serious anger issues. Or you might've done something that really riled them up.

There are many things to consider....

How close the person is to you, whether they've sinabi similar things to you before, whether they actually ever did any of those nakaraan things, what their history of violence/crimes/behaviours is...

You can confide in someone close to you and then see what payo they can offer.