Co znaczy „na kogo przypadnie, na tego bęc” ?

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It's a line from a nursery rhyme that chlorekmiedzi2 mentioned.
It's used in the situation when we have to decide who is first to do something.

I will try to explain.
"na kogo wypadnie na tego bęc"

Wypaść na kogoś — when you say this rhyme at the same time you point at some people and when you're done you point at one person only and this person is the first one to do sth. Eg. in my video the teddy I stopped at was the llama and I can say "wypadło na lamę.".= The fate decided that it is llama to do something. 🙂
Or eg. I talk with my friend about who should call to order some pizza. She says: "I will toss a coin, heads – I win and you have to call". She tosses and it's heads so it means I have to call. She can say "wypadło na ciebie". *introvert mood*
So literally it means "it fell out on you" but the real meaning is that "you do it because the fate decided so".

Na tego bęc — so, continuing the phrase "the person on whom it falls out", the next part is "thud on him".
It's like a pun because, as I mentioned "wypadać" literally means "to fall out" and when something falls against the floor it makes this sound "bump" or "thud" — this is "bęc".
"Boom" is more connected with an explosion, so the Polish equivalent wouldn't be "bęc" but "bum".

Sometimes when we see little children falling over in a cute way (and when it's not dangerous for them ofc) we might say "oj! bęc!" which is like "oops, you fell". :)