The man charged with breaking into Speaker Nancy Pelosi's office during the Capitol riots insists he wasn't calling her a "bitch" on paper -- he was just calling her his "biatch" ... so it's not nearly as bad as the feds are making it sound.

Richard Barnett is set to appear Tuesday in court to make that very argument for why he isn't a dangerous threat ... and should be cut loose pending trial. But, before he tells it to the judge, his attorneys filed docs, explaining this sort of slang defense.

In the docs, obtained by TMZ, Barnett's lawyers say prosecutors' claim he wrote "Nancy, Bigo was here, you bitch" on a sheet of paper on her desk is false. Instead, they assert he wrote ... "Hey Nancy Bigo was here biatd." To them, that last word was supposed to be, "biatch." If nothing else, he's guilty of poor penmanship.

The attorneys argue Barnett combined the "c" and the "h" at the end there -- and that he was going for the slang version of "bitch" ... which they note is not as offensive, and might even be considered endearing in some contexts.

Basically, his attorneys are saying he was just clowning around -- and didn't actually intend to harm and/or threaten her ... despite making himself at home and having his run of the place.

Barnett was previously denied bail ... but a lot of alleged Capitol rioters have been released from jail since then, based on rulings they do not pose flight risks or other dangers. With that in mind, Barnett's lawyers are hoping to get their client out ASAP.