Portraits of Napoleon and Nelson etc showed the same pose. My understanding was that, until the advent of the sewing machine, one was obliged to hold up one's trousers. The hand in the jacket, (and remember "low-risers" or "hipsters" had yet to become fashionable) was essential in order to distinguish early portraiture from early pornography. Lord Admiral Nelson, apparently very much annoyed by this pose requirement is reputed to have said to the artist: "All this fuss over a little thing like that !"

Bob H