I don't know even 1

Origin

"1-do moreugetda" ("I don't understand one bit") comes from episode 54 of MBC's "Real Men," broadcast on April 13, 2014. Henry Lau, a Canadian-born singer, wrote on a training answer sheet "I don't understand 1 bit of what was said," and the shot aired as-is (Segye Ilbo coverage). His Korean was shaky, so he rendered "hana-do" (not one bit) using the numeral 1, and the mistake became a catchphrase.

One detail is widely misremembered. Henry himself read the sentence as "hana-do," not "il-do." It was purely a spelling error at the source; afterward people began reading the characters on screen literally as "il-do," and that pronunciation hardened into a new expression. The catchphrase is one mutation further along than the original.

Spread

It spread rapidly through Korean communities and social media right after broadcast and settled into everyday speech as an emphatic way of saying "not at all." Its range widened until the numeral 1 functioned almost like an adverb meaning "not in the slightest": "1-do eopda" (none at all), "1-do an haetda" (didn't do any of it), "1-do an gunggeumhae" (not curious in the slightest). It is a rare case of a line from one broadcast spreading as far as a grammatical unit.

It also drew the usual complaints about language degradation, since the spelling breaks standard orthography. Part of why it caught on, though, is that "1-do" simply looks more emphatic on the page than "hana-do," and usage continued regardless of the normative argument.