I saw this entry from the Appendix Probi and can't seem to decide what it is exactly and what it means? From looking at it, it has something to do with walking from the 'calco' part, but not sure about 'stegis'. Is this a lost idiomatic expression?
This is a Greek word χαλκόστεγος "bronze/copper-roofed", and seems to refer to the imperial palace at Byzantium. Some googling found these references:
(Latin) "The house of Emperor Anastasius ... was previously called χαλκόστεγος ... because of the bronze/copper roof"
(Greek) "the roof of the palace at Constantinople, covered with bronze/copper"
As far as I can tell, nobody is entirely sure.
I've found no attestations of this particular word anywhere except the Appendix Probi. TKR mentions χαλκόστεγος "bronze/copper-roofed", but it doesn't seem to have really caught on in Latin contexts—I've only been able to find it as a direct quote from Greek.
Kerkhof gives the translation "bronze roof beams" in passing, presumably taking that interpretation (χαλκός + στέγος) and making it into a noun, but I can't find any further discussion of this. Some people do correct that line to chalcostegis, presumably to align it with χαλκός, but Powell disagrees.
Barnett, on the other hand, raises "the possibility that calcostegis and septidonium (entries 12 and 13, after the syncopes) are the debris of an original reference to the seven-stringed lyre, mentioned in a quotation from the Alcestis of Euripides". However, I can't find any discussion of a seven-stringed lyre in that work; there are a few mentions of lyres, but all with general terms like λύρα. And Barnett doesn't expand on this any further or explain what a calcostegis would be in a musical context—I wouldn't be too surprised if στέγος had a technical meaning in luthiery (like the modern use of "bridge"), but if it did I haven't been able to find any.