I researched it, because this isn't a word that Americans use often. Here is what the Merriam-Webster dictionary says: "Etymologists aren't exactly sure of the origins of the verb dicker; however, there is a probability that it arose from the bartering of animal hides on the American frontier. The basis of that theory is founded on the noun dicker, which in English can refer to a quantity of ten hides. That word is derived from decuria, the Latin word for a bundle of ten hides, and ultimately from Latin decem, meaning "ten". In ancient Rome, a decuria became a unit of bartering. The word entered Middle English as dyker and eventually evolved to dicker. It has been posited that the verb emerged from the bargaining between traders over dickers of hides, but not all etymologists are sold on that idea." Good luck!

DICKER (verb) : To bargain, haggle or negotiate over a sale; To barter. DICKER (noun) : (obsolete) A unit of measure, consisting of 10 of some object, particularly hides and skins. Etymology From Middle English DIKER (“measure of ten”), from Late Latin DACRA (“a dicker”), from Latin DECURIA (“a ten of something”), from DECEM (“ten”).

dicker- (informal especially American English) to argue about or discuss the details of a sale, agreement etc dicker about/over smth Origin: the noun "dicker" appeared in 11 century, meant “ the group of ten (animal skins)” Later was used as a verb in the exchanging process of animal skins for other goods