[sent; these forms only; fixed WO]
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⇒ (of a person) s.o. goes or runs somewhere and returns in a very short time, very quickly (often used as a command):
- I <he etc> will be back (will do sth.) before you know it (before you can turn around, in a flash, in a jiffy, in two shakes of a Iambus tail).
♦ [author's usage] "Побегу, доложусь, дядя Коль, начальство порядок любит. Я бегом: одна нога там, другая - здесь" (Максимов 1). Til go and report, Uncle Kolya. The bosses like order. I shan't be long. I'll be back before you know it" (1a).
♦ "Иди тащи сюда свою роту". - "Я знал, Савва..." - "Иди, иди, а то раздумаю". - "Одна нога здесь, другая там..." (Максимов 2). "Go ahead and bring your horde in here." "I knew you would, Savva..." "Go on, before I change my mind." "I'll be back before you can turn around" (2a).
Большой русско-английский фразеологический словарь. — М.: ACT-ПРЕСС КНИГА. С.И. Лубенская. 2004.