The phrase "indelible ink" means a type of ink that cannot be easily erased, removed, or washed away. It leaves a permanent mark that cannot be easily changed or undone. Full definition
Usually no one writes them down, but they get deeply scrawled in indelible ink on our cerebrum. (psychologytoday.com)
Using a pen with indelible ink, write on the collar your name, address and an emergency number where you can be reached. (articles.orlandosentinel.com)
Examples include ballot papers being spoiled by weather due to the flimsy construction of polling stations and poor quality indelible ink (in which voters were required to dip their index fingers on leaving polling booths) resulting in some double - voting. (blog.politics.ox.ac.uk)