Take the gilt off the gingerbread
Remove an item's most attractive qualities.
This phrase has nothing to do with being guilty of anything of course - 'take the guilt off the gingerbread' is just a misspelling. The word is 'gilt', which refers to a thin layer of gold. Gingerbread is a form of cake and, although the association of the two words seems a little odd, gingerbread cakes were in fact gilded for festivals and other special events in the Middle Ages.
The word gingerbread has been recorded in English since the 13th century; it was originally a form of simple cake flavoured with treacle and, not surprisingly, ginger. It was, and is, frequently made as a form of biscuit, by rolling out the dough and cutting it into shapes - men, animals, letters of the alphabet etc.
Gold can be hammered to a minute thickness to form gold leaf. This can be 'gilded' to many different surfaces, including cake, and is harmless when eaten in small quantities - hence its use as a culinary decoration.
The expression 'taking the gilt off the gingerbread' isn't found in print until the 19th century, but the practise of gilding gingerbread cakes was probably in place well before that, as 'gingerbread' has been used as an adjective meaning 'showy and insubstantial' since the 17th century. Gingerbread without its casing of gold leaf was a rather humble offering - often little more than flavoured stale bread, and not likely to attract a reputation for showiness. Using present-day parlance, ungilded gingerbread was more minging than blinging. An early reference to that disparaging usage of gingerbread is found in The History of the tryall of Cheualry, 1605:
Anticke! thou lyest, and thou wert a Knight of ginger-bread: I am no Anticke.
The first evidence that I can find in print of 'take the gilt off the gingerbread' is from The New Zealand newspaper The Lyttleton Times, February 1854:
To borrow a homely expression, it was determined in taking the gilt off the gingerbread.
The 'homely expression' reference indicates that the phrase was already known, in the Antipodes at least, by 1854.
It has been suggested that 'take the gilt off the gingerbread' derives from the ornately carved decoration on ships and buildings. This decoration was known as gingerbread-work and was referred to in print by Tobias Smollett in The adventures of Roderick Random, 1748:
Lookee, - if you come athwart me, 'ware your gingerbreadwork. - I'll be foul of your quarter, d--n me.
Gingerbread cakes had been decorated with gold for centuries before the naming of gingerbread-work and it is clear that the gilded decoration was named after the cake rather than the other way around. 'Take the gilt off the gingerbread' wasn't coined until after both meanings were known, so it could be derived from either. None of the early uses of the phrase relate to ships. It seems more likely that the phrase derives from the 'cake' context.
Sadly, gingerbread men (now often politically corrected to 'people') have had the gilt taken off them and are now usually clad with icing sugar.