The following explanation of Nivul Peh was taken from:
Nivul Peh Rabbi Gideon Weitzman
Adar I 6, 5771
:1) Could I safely say that nivul peh is speech which illustrates sexual conduct or that insites lust in the average individual?2) Is speech that reference to excretory organs or excrement also nivul peh?If not, what isur is it? where it it written that reference to excretory organs or excrement is assur?I’m not asking bc I have doubt, but because I want a source.
Answer:Shalom U'verachah,The Ramban explains that Hebrew is lashon hakodesh because it does not have names for the reproductive organs, thus if we strive to be holy we should not explicitly name these organs, unless neccesary for education or in a medical context etc. but not just in every day speech. Any discussion of inappropriate matters of modesty can be considered nibul peh. This includes swearing and cursing. The Gemara even speaks about how the Torah added words so as to say the animal that is not clean and not call it an "unclean animal." Kol Tuv