Summary: The prefix *ob-*, besides meaning "against" or "towards," can also act as an intensive prefix. An intensive prefix can effectively be translated as "thoroughly" to highlight its emphatic function. The intensive nature of the prefix *ob-* shows up in such words as [*ob*]tuse and [*ob*]durate. An example to help remember this intensive prefix is the word [*ob*]tain, for when you [*ob*]tain something you "thoroughly" hold onto it.
