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. [early 18C–19C] (also callabash) the human head.

2. [mid–late 19C] (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].

In compounds

calabash cover (n.)

[late 19C] (US) a hat.