Sale has ended but you can still get this app.
Download Old English Dictionary Now

Official Description

This is an offline, fully searchable version of the public domain lexicon by Bosworth and Toller for Old English (Anglo-Saxon). This is the most complete Old English dictionary available on or off-line.

Features include:

(1) Fully indexed list of headwords, searchable by prefix.
(2) Custom Old English keyboard.
(3) Full text search by keyword on the complete definition entries.