On form 1116 Foreign Tax Credit form there are references to HTKO yet the acronym is never defined. Googling leads me to believe that it stands for "High Tax Kick-Out". But I haven't been able to find a solid definition of what it really means. The official page at the IRS site with directions for filling out this form just references passive income that is "high taxed" for example:
If you have passive income that is treated as general category income because it is high taxed, use a separate column in Part I. Enter "HTKO" on line g of Forms 1116 for passive category income and general category income.