(КАК (БУДТО, СЛОВНО, ТОЧНО)) АРШИН ПРОГЛОТИЛ coll, humor(как etc +) VPadv (variants with как etc) or predic with subj: human (all variants)past onlyfixed WO
(a person who) has unnaturally straight posture (may refer to a permanent or a temporary characteristic)
straight as a ramrod
ramrod-straightstiff as a poker(one) looks like he has a broomstick (a poker) up his butt.
"Я вот чего хотел-то: разводец бы нам организовать... Да что ты молчишь?» - как будто удивился (Дмитрий)... А она (Маруся) стоит и крикнуть не может... Ведь её - Дмитрий! Как же так?.. «Ну вот, будешь теперь стоять, как аршин проглотила...» (Суслов 1). "What I wanted was this: we should get a divorce.... Why are you so quiet?" he (Dmitry) asked, as if he were surprised....And she (Marusia) stood there and couldn't cry out....It was her Dmitry! How could this be?..."So now you just stand there, stiff as a poker" (1a).
...Аршин» is an old Russian measure of length (0.71 meter). The term also refers to a stick or ruler of that length (formerly used by merchants, tailors etc).

Большой русско-английский фразеологический словарь 

А43 →← А41

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