“No biggie“ is used to say that something is not a big deal; you shouldn’t worry about it. This phrase is mainly American. The positive form of this phrase is “biggie“ meaning big deal. The word “biggie“ in British slang is what a child calls his poo! Now can you imagine what a British child would have in mind when you say “It’s no biggie” to him/her? 😀
Synonyms: no problem, no sweat, no worries.