adverb, noun
A yojijukugo meaning constantly, around the clock. The literal breakdown is 二 (two) × 六 (six) = 12 hours, doubled for a full 24-hour day. Used in both adverbial and noun roles.
彼は二六時中仕事のことを考えている。
He thinks about work night and day.
二六時中スマホをいじっているのはよくない。
It's not good to be on your smartphone all the time.