UK, 1923—the north-east corner of Hyde Park, London, near Marble Arch, where public speaking and debate are traditionally permitted with minimal restriction—by extension: any location where public speaking and debate are permitted or encouraged
UK, 1867—a disastrous or particularly unpleasant year—Latin, literally ‘a horrible year’—coined after Latin ‘annus mirabilis’, literally ‘an extraordinary year’
grim and ironical humour—UK, 1860, in reference to the practice of public executions—UK, 1870, as a loan translation from German ‘Galgenhumor’, in the context of the Franco-Prussian War
in the phrases ‘(as) regular as pig tracks’ (1853) and ‘(as) common as pig tracks’ (1854), the plural noun ‘pig tracks’ is an intensifier—Southern United States
the killing of a woman or girl by a man—French ‘femmicide’: 1839; 1854 in the sense of a man who has killed a woman—English ‘femicide’: 1801; 1827 in the sense of a man who has killed a woman
Yorkshire & Lancashire (northern England), first half of the 19th century—the evening of 30th April (May Eve), on which people traditionally indulged in mischievous pranks
New Zealand, 1970—at full speed—perhaps after ‘flat out’ (i.e., with the maximum speed or effort) and after ‘quicksticks’ (i.e., quickly, without delay)
the solution to a problem is very straightforward and easy—UK, 1901—supposedly said by Sherlock Holmes to Dr. Watson—but this phrase is not (in this form) found in any of Arthur Conan Doyle’s stories