Green’s Dictionary of Slang

calabash n.

[Persian kharbuz, or kharbuza, meaning melon, or watermelon; ult. Arabic khirbiz, melon or kirbiz, pumpkin or gourd]

1. (also callabash) the human head.

[US]Boston News Letter 27 June 2/2: You have in the cavity of your Callabash a viscid juice [DA].
[UK]Grose Classical Dict. of the Vulgar Tongue ms. additions n.p.: Calabash the Head. I’ll Split his Calabash.
[US]R. Waln Hermit in America on Visit to Phila. 2nd series 31: There was a chance of having his peepers plumped by a bully, or his calabash cracked by a Watchman!!
[US]J.C. Neal Charcoal Sketches (1865) 97: ‘Take care of your calabash then,’ was the grinning response.
[US]Alta Calif. 6 Apr. 2/3: Brady says his wife struck him over the calabash with a brass candlestick [DA].
[US]Schele De Vere Americanisms 121: calabash, by far the most frequent use made of the word is, as a cant term, for a weak and empty head.

2. (Aus., also calibash) a promissory note or IOU [the image is of the essential worthlessness of such notes, which were no more valid as money than had they been written on a calabash or gourd-shell].

[Aus]Sydney Morn. Herald 6 Aug. 3/5: [Y]our correspondent had about fifteen of these ‘calabashes,’ (so termed by the colonial slang), all amounting to not more than £7, drawn on one of our M.C.’s.
R.D. Barton Reminiscences of an Aus. Pioneer 159: Everyone was paid by orders, ‘calibashes’ we used to call them, drawn on himself by the person paying.
[Aus]Smith’s Weekly 12 Nov. 6: Mention of ‘shin-plasters’ recently in ‘Smith’s Weekly’ suggests a mention of ‘Calabash’. This was a form of currency in the early days, and was originally an order for a small amount, drawn upon some agent of the drawer and payable at various dates after presentation. Finally a calabash became an IOU for sums under one pound [GAW4].

In compounds

calabash cover (n.)

(US) a hat.

[US] (ref. to 1886) W.J. Abbott Blue Jackets of 1918 269: ‘Where will you carry the despatch?’ [...] ‘In my calabash-kiver, massa,’ he answered, pointing to his thick, woolly head [HDAS].