Green’s Dictionary of Slang

1. the human head [play on nut n.¹ (1)].

c.1895		C. Osborne [perf. Herbert Campbell] ‘Dada’s Baby Boy’ 🎵 She stands on her filbert, and gracefully falls / Jerusalem, how she Lord Mayors.
1903		Hull Dly Mail 10 Sept. 4/1: She was bleeding profusely from the mouth and nose. Clegg [...] exclaiming, 'Emma, Emma, hold up your -- filbert' (meaning her head).
1914		Ade ‘The New Fable of the Intermittent Fusser’ in Ade’s Fables 59: He was on the waiting list for the Nut Club. Our Old Friend was flooey in the Filbert.
1920		Ade Hand-made Fables 292: Experts tell us that this is the first Sign of a general breakdown in the Filbert.
1936		J. Curtis Gilt Kid 198: Get that into your old filbert.
1981		J. Sullivan ‘A Slow Bus to Chingford’ Only Fools and Horses [TV script] Use your old filbert.

2. a fashionable dandy [pun on knut n.; esp. in song ‘Gilbert the Filbert/Colonel of the Knuts’, by Arthur Wimperis (1874–1953) featured in a 1914 version of The Passing Show].

1914		Wimperis & Finck [perf. Basil Hallam] ‘Gilbert the Filbert’ 🎵 I'm Gilbert the Filbert the Knut with a K / The pride of Piccadilly the blasé roué.
1915		Grey Brigade 20 Nov. 4/1: Agony Column — Young subaltern recently divorced is not averse to making the acquaintance of passionate actress. When not in camp wears soft hat, stock tie, pink socks and Oxford Shirt. The Filbert.
1915		West Briton & Cornwall Advertiser 1 Nov. 4/3: Ferdy Fullobounce had deigned to honour a seaside hydro with his exquisite presence. He was a perfect ‘filbert,’ a true sample of the ‘cuff and collar brigade’.

3. (US) a crazy person; a clownish person [play on nut n.² (1)].

1920

Film Fun 24 Apr. 1: All the FILM FUN filberts have it! They win!