Wiktionary
a. That cannot be taxed; on which tax is not paid.
Usage examples of "untaxable".
Even the president of a bank had financial limitations, but John Larimer had a large source of very liquid and very untaxable income and when he stopped being a family man, he could enjoy his nights with Fifi, Honey, Pussy and Snookums, who were very expensive playmates.
What this Monroney guy was offering was the first sensible proposition Malachi had seen since coming to this crazy country, an dover the next two and a half years, Malachi made more money than he had ever dreamed of making in ten careers in the Army--in cash, nice, green, untaxable cash, complete with accommodating Chinese bankers in Cholon who could move things around a network of extremely discreet banks all over the world.
There were other, more subtle benefits, too, involving tax write-offs and untaxable trades for existing, surplus museum stock.