/ob/ prefix Obligatory. A piece of netiquette acknowledging that the author has been straying from the newsgroup' s charter topic. For example, if a posting in alt.sex is a response to a part of someone else' s posting that has nothing particularly to do with sex, the author may append "ObSex" (or "Obsex") and toss off a question or vignette about some unusual erotic act. It is considered a sign of great winnitude when one' s Obs are more interesting than other people' s whole postings. [Jargon File]