AskHistorians

When you are infantry, pretty much the hardest thing in your training will be to learn to suck it up and advance under fire.

Genuinely. The training for marching and shooting takes a few weeks, but getting the idea through someones head that they must not allow their body to take control and freeze when advancing into enemy positions is much harder than learning to walk in lockstep.

That's why you drill, drill, drill, because you're literally drilling those movements into your body, so it does them automatically instead of freezing under fire.

Oh wow, I was gonna say that's where the saying comes from, but "you must not stay still under enemy fire" is not a saying in English.

It is in Finnish.

Ei saa jäädä tuleen makaamaan, is something at least males hear from a very early age on. Like even outside any sort of military context. Basically where Churchill said "keep buggering on", we do things from the negative and instead of saying "keep pushing forwards" it's "don't stop pushing".

https://en.m.wiktionary.org/wiki/j%C3%A4%C3%A4d%C3%A4_tuleen_makaamaan

jäädä tuleen makaamaan (literally, “to stay lying in gunfire”)

(intransitive, idiomatic, chiefly in the negative) to not keep pushing forward, to give up trying

Ei saa jäädä tuleen makaamaan.

   >     One must keep pushing forward

Tldr the point here being it's literally the most logical option at that point, so even when you know you're gonna get dowsed with MG fire, you just hope they'll be aiming for someone else for that split second when you move.