Acidic tendency of copper(II) ion vs. calcium(II) ion
Cu2+ has an incomplete 3d orbital (4s0 3d9) and can therefore be included in the chemistry of transition metals. As it can be coordinated (form dative covalent bonds) with ligands to create complex ions, for example H20 in hexaaquacopper(ii), it acts as a Lewis acid.

But in terms of Brønsted–Lowry theory, perhaps the textbook is referring to the fact that when transition metals are dissolved in water they can donate an H+ ion to another H2O molecule to form H3O+. This can be observed with Fe3+:



[Fe(H20)6]3+ + H2O -> [Fe(H20)5(OH)]2+ + H3O+

The complex ion is acting as an acid by donating a hydrogen ion to water molecules in the solution. The water is, of course, acting as a base by accepting the hydrogen ion.