Hello. What does it mean "Ctopudoff"? Explain me, please.
the word cтoпyдoвo is slang used by the youngsters. Cтoпyдoвo is an adverb that comes from two words: cтo (hundred) and пyд (pood).
Пyд is a measure of weight equal to 16 kg. This word was widely used in Russia in the past, before the Lenins decree had adopted the metric system of measures and weights.
Thus, cтoпyдoвo could literally mean one hundred poods. In reality, this means 100%, definitely, guaranteed.
source -
https://ochenporusski.com/stopudovo/I've never heard such long explanation of my nickname

And everything is almost true except "does not mean he will keep his promises"
Word "cтoпyдoв" has nothing in common with keeping promises. It is just means definitely yes or 100% true.
This word is also used among people of different ages, not only youngsters.