Green’s Dictionary of Slang

sailor’s slang, nautical jargon.

1711		Medleys 29 Jan. (1712) 186: I perceive nothing even of the Merchant in him but His Altumal Cant, a Mark of his poor Traffick and Tar-Education .
1771		Encyc. Britannica I 129/1: Altumal, a term sometimes used to denote the mercantile style or dialect.
1753		Chamber’s Cyclopedia Supplement n.p.: Altumal, a term used to denote the mercantile style, or dialect. In this sense, we meet with altumal cant, to denote the language of petty traders and tars [F&H].