The abover definition is completely false, because the federal income tax doesn't discriminate between sources of income from government occupations and private labor. See e.g. 26 U.S.C. § 61. Otherwise, how would the federal government be able to fund its operations?
If a citizen refuses to pay this tax, they will often be arrested at point of gun, and put in jail - making it a true definition of slavery.
A construction worker gets his pay from non federal contracts in the private sector of on of the several states (such as California). His "income" does not come from a FEDERAL source nor any of the few named in section 861 & operative sections of the Internal Revenue Code. Therefore, his compensation for labor done within that state (geography is a deciding factor too -- jurisdictional) is NOT taxable.