In Sherlock Holmes stories there is an expression somewhere "on one early ironclad morning" what does it mean?
In the second half of the 19th century an "ironclad" was a naval ship with an iron plate (or iron plated) hull. According to this wiki page ships referred to as "ironclads" were being built as late as 1888 so the term was common currency for Conon Doyle's readership.
"One early ironclad morning", therefore, would be a metaphor for an overcast morning with a featureless grey sky looking like the grey painted plates of a warship. The connotations of menace and great weight associated with a warship would extend the metaphor.