A British term, bordering on slang, meaning to remodel or restore an old building without proper knowledge of its historical character, or without remaining faithful to its original qualities and uniqueness. It may imply that the person doing the remodeling has more money than common sense.
This comes from Edmund Beckett Denison, who was named Lord Grimthorpe in 1874. He is probably best known for designing Big Ben, but the word grimthorpe entered the common language because of the work he did on St. Albans Abbey. When the St. Albans Abbey Reparation Committee ran out of funds, Sir Edmund Beckett stepped in and volunteered to fix it up at his own expense -- as long as the work was done according to his own designs and under his supervision. As you have no doubt guessed, he grimthorped the project. He 'messed up' the roof, the west front, and some of the windows. There was much criticism, but for the most part Lord Grimthorpe's changes remain today. He also worked on some other churches in the area, including St. Peter's, with much the same effect.