...the figurative, which I suspect just as quickly completely superseded it altogether, although putting a date to that would be tricky.
Indeed. But the literal birthday suit ceased to be an important thing round about 1800. Before that, for hundreds of years men had as a matter of course bought, borrowed (or, occasionally, hired) the most lavish and spectacular outfits they could afford to go to court. For most of the 18th century, the outfits men wore to royal birthday courts - the most lavish court occasions outside of royal weddings et cetera - were as dazzling as the women’s: they were made of the same types of brocaded silks and figured velvets, and like the women’s were embellished with elaborate embroidery and set off with costly lace ruffles, jewelled buttons, and what-have-you. These being public occasions, the outfits worn were critiqued in the popular press and in letters and diaries, just like Hollywood red-carpet events today, and men’s outfits got as much attention as women’s:
1722: Lord Essex has a silver tissue coat and pink color lutestring waistcoat ... several had pink color and pale paduasoy coats, which looks prodigiously effeminate’ (Sarah Byng Osborn)
1739: ‘my Lord Baltimore was in light brown and silver, lined throughout with ermine’ (Mrs Delany)
1753: ‘the Duke of Portland...very fine...dark mouse-coloured velvet embroidered with silver; Jenny Glegg’s work, and the finest I ever saw: the waistcoat Isabella satin, embroidered the same as the coat’ (Mrs Delany)
But in the 1790s all this changed. Hoop-petticoats fell out of fashion, but continued to be de rigueur for ladies at court - and very bizarre they looked too with the high waistline of the period - so while ladies’ court dress continued to be magnificent it was no longer of fashion interest. Something similar happened to men’s court coats; they followed non-court fashion up to a point, becoming simpler in cut and made in unpatterned materials in dark colours - black, brown and ‘prune colour’ being common. The embroidery became simpler and less blingy, and sometimes was even left off the coat altogether and restricted to the waistcoat. But embroidered or not, as the 19th century dawned the coat retained the cut of the last decade of the 18th, with the ‘swallowtail’ curving edges just meeting in front. The legwear was still breeches, now an archaism. The whole outfit was a fossil, no longer fashion news. That being so, a gentleman who went to court once only would most likely hire a court suit; it would be a daft extravagance to buy one as it couldn’t be worn for anything else. Anyone who had to attend regularly would buy one and stick with it until it looked shabby or until the Lord Chamberlain updated the dress regulations.
So it’s my guess (only my guess) that by about 1810, the literal sense was only being used in the kind of sentence that begins ‘In my young days we....’