Off your rocker

If you are 'off your rocker' (a nickname for a rocking chair), it means whatever you're saying is completely wrong. It most likely originated with senior citizens, who often use rocking chairs.

Example:
-I may be completely off my rocker here, but I think your coworker has a crush on you.
-No, you're not off your rocker! I think so, too.

Feeling out of sorts

If you're 'feeling out of sorts,' it means that you're not quite feeling like yourself. You feel a little off, and perhaps a little sad.

Example:
-How are you feeling today?
-Well, I'm a little out of sorts. I'm not sure what's bothering me.

To go off on someone

'To go off on someone' means to tell someone how angry you are about something they did.

Example:
-So what did she say?
-She totally went off on me, saying how immature and irresponsible I am.

Off the top of my head

'Off the top of my head' is used to refer to the first thing a person can think of.

Examples:
-Do we need anything at the grocery?
-Not off the top of my head.

-What do you want for your birthday?
-I can't think of anything off the top of my head.