Americans use "off limits" and "out of bounds" interchangeably and about equally - with the exception that only "out of bounds" has a specific meaning in sports. Actually, I always thought that "off limits" came to us from the UK during WW2 while "out of bounds" originated in the US.

Does "suck" have some connection to fellatio? I dunno - I remember from my childhood that it was what you did to a popsicle or a lollipop (also called a sucker), and fellatio wasn't on my personal horizon at the time.