The phrase "iron fist" refers to someone who rules or controls with strictness and force, without any tolerance for disobedience or opposition. It implies using power and authority in a harsh and uncompromising way. Full definition
«Wielding his signature Engine Blade, Noctis is ready to deal heavy hard - hitting blows as he takes on the likes of Yoshimitsu, Devil Jin, Jack 7, and the rest of the Tekken 7 fighters when he enters the King of Iron Fist Tournament in spring 2018!» (stevivor.com)
These «terrible» little babies were on a mission to break and control mommy and daddy, to «manipulate» from day 1 and parents had to rule with iron fists and learn to ignore those «manipulative» cries. (littleheartsbooks.com)
In 2015, Nobi won EVO and the King of Iron Fist Tournament for Tekken 7. (unifiltekken.com)