막무가내가 · 막무가내를 (often 막무가내로, 막무가내예요)
being impossible to reason with, stubbornly insisting no matter what
Example
아이가 장난감을 사 달라고 막무가내로 떼를 썼어요.
The child threw a tantrum, flatly insisting I buy him a toy.
Word parts
막 none+무 without+가 can+내 how (to cope)
Literally 'there is no way at all to deal with it' (莫 'none', 無 'without', 可奈 'how one could cope') — someone nobody can do anything with.
Good to know
Pronounced [망무가내]. Usually 막무가내로 + 우기다/떼를 쓰다/조르다, or as a predicate: 막무가내예요.