The latter had never been "underdrawn"

Source : Wuthering Heights, by Emily Bronte.

Extract:

One end, indeed, reflected splendidly both light and heat from ranks of immense pewter dishes, interspersed with silver jugs and tankards, towering row after row, on a vast oak dresser, to the very roof.The latter had never been under-drawn: its entire anatomy lay bare to an inquiring eye, except where a frame of wood laden with oatcakes and clusters of legs of beef, mutton, and ham, concealed it.

What does the word "under-drawn" mean in this context?

It should be quite clear from the context, the passage is talking at that point about the roof and tells you that because it has never been 'under-drawn' you can see its whole structure. Therefore there is no ceiling, you can see the rafters.

Underdraw . to line the underside of (a structure, as a floor) with plasterwork, boarding, or the like.
Dictionary.com

Where the structure is described as being concealed, it is concealed by

a frame of wood laden with oatcakes and clusters of legs of beef, mutton, and ham

It means something akin to this, if likely not quite so grand.

a version is shown in this image which is a simple stick suspended from the beams, but shown how the things stored on it (to keep them away from vermin) could be bulky and stop you seeing the roof beyond.