The American Heritage® Dictionary of the English Language: Fourth Edition. 2000.
ob
PREFIX:
Inverse; inversely: obcordate.
ETYMOLOGY:
New Latin, short for obvers, obversely, from Latin obversus, past participle of obvertere, to turn toward : ob-, toward, against (from ob, toward, against, before; see epi in Appendix I) + vertere, to turn; see versus.