hysterical reasons
(Or "hysterical raisins") A variant on the stock phrase "for historical reasons", indicating specifically that something must be done in some stupid way for backward compatibility, and moreover that the feature it must be compatible with was the result of a bad design in the first place. "All IBM PC video adaptors have to support MDA text mode for hysterical reasons." Compare bug-for-bug compatible.
[The Jargon File]