Green’s Dictionary of Slang

b flat n.

a bedbug.

1836		Tait’s Mag. Nov. 694: The author’s greatest suffering arose from Carlist fleas, and those insects known in polite life by the delicate name of b flats [F&H].
1853		Dickens Household Words xx 326: A stout negro of the flat back tribe – known among comic writers as b flats.
1860		Hotten Dict. of Modern Sl. etc. (2nd edn).
1868		Brewer Dict. of Phrase and Fable (1903) I 80/1: B flats – bugs. The pun is ‘B’ (the initial letter), and ‘flat’, from the flatness of the obnoxious insect.
1874		‘Old Calabar’ Won in a Canter III 99: ‘l’m bitten all to pieces — eaten up — look at my face.’ ‘Well, Sir, there is no denying as them B flats has been at you’.
1881		T. Hughes Rugby Tennessee 58: An insect suspiciously like a British B flat.
1890–1904		Farmer & Henley Sl. and Its Analogues.
1900		E.H. Babbitt ‘College Words and Phrases’ in DN II:i 22: b-flat, n. Bedbug.
1909		J. Ware Passing Eng. of the Victorian Era 13/2: B Flat (Peoples’) [...] an intimate insect (now rapidly being evicted by a survival of the fittest), which has been too fatally associated with the family of Norfolk Howard.