BROADCASTS.com

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 obtuse and obdurate. An example to help remember this intensive prefix is the word obtain, for when you obtain something you “thoroughly” hold onto it.
Like this? Build a competent vocabulary with Membean.