〜ようものなら
if (you dared to), then (disaster)
Verb volitional form + ものなら
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ようものなら says that if someone did something, even a little, the result would be terrible: 少しでも遅れようものなら、部長に叱られる, "if I'm even a little late, the manager tells me off".
It uses the volitional form (遅れよう, 話そう, 転ぼう) plus ものなら. The first half is a small or ordinary action; the second is an exaggerated bad consequence.
It's often used to describe strict people or dangerous situations, with a slightly humorous, exasperated tone: 母に口答えしようものなら、ひどく怒られる, "if I ever talk back to my mother, I get a real telling-off".
Compare ものなら after a potential verb, which is a wistful wish. With the volitional, it's a warning.
Examples
母に口答えしようものなら、ひどく怒られる。
If I ever talk back to my mother, I get a real telling-off.
少しでも遅れようものなら、部長に叱られる。
If I'm even a little late, the manager tells me off.
彼に秘密を話そうものなら、すぐにみんなに広まる。
If you ever tell him a secret, everyone will know in no time.
こんな所で転ぼうものなら、大けがをする。
If you fell somewhere like this, you'd be badly hurt.
試験に落ちようものなら、親に何を言われるか。
If I failed the exam, I dread to think what my parents would say.