Ob- /ob/ pref.

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.

--The Jargon File version 4.3.1, ed. ESR, autonoded by rescdsk.

Ob- (?). [L. ob, prep. Cf. Epi-.]

A prefix signifying to, toward, before, against, reversely, etc.; also, as a simple intensive; as in oblige, to bind to; obstacle, something standing before; object, lit., to throw against; obovate, reversely, ovate. Ob- is commonly assimilated before c, f, g, and p, to oc-, of-, og-, and op-.

© Webster 1913.