In the dictionary, under the origen of the word, what does "bef. 900" mean?

Answer 1

"bef 900" means :"before AD 900"

That is a common abbreviation in ENGLISH dictionaries, because AD 900 was the very end of the reign of King Alfred the Great (of Wessex) -- an important turning point in England's history and, generally considered the dividing line between "Early Old English" and "Late Old English"

In other words, if a word is listed as "bef 900" that means it goes all the way back to an early Old English form.

In case in helps, this page should show you a conventional division of the periods and SUB-periods of the history of the English language (not just "Old English", "Middle English" and "Modern English" but periods WITHIN each).
http://books.google.com/books?id=SIzqSvfvwp0C&pg=PA16&lpg=PA16&dq=900+alfred+old+english&source=web&ots=oM_IHmn86V&sig=mlpw_HpgogaZwi6WRIogOprWQK8

Answer 2

Before the year 900.