This is an offline, fully searchable version of the public domain lexicon by A. L. Mayhew and Walter W. Skeat, A Concise Dictionary of Middle English From A.D. 1150 To 1580 (1888). Interested in reading Chaucer or the Wycliffe Bible? This is the lexicon you will need.

Features include:

(1) Fully indexed list of headwords, searchable by prefix.
(2) Full text search by keyword on the complete definition entries.
(3) Custom Middle English keyboard.
(4) Does a licence check only on the first start-up and thereafter is completely offline.