Green’s Dictionary of Slang

mellish n.

[? Lat. mel, honey, thus the image of money as a ‘sweetener’ and/or the golden colour]

a sovereign.

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[UK]‘Jon Bee’ Dict. of the Turf, the Ring, the Chase, etc. 209: Mellish, or Miel-ish a sovereign. Probably from Miel, honey, i.e. a ‘sweetener of life.’.