Three Arabic letters saad waaw and meem (م و ص) are the roots of the term Soum, which basically means to stop, come to an end, or discontinue. Let me present some examples where the word is employed in its basic meaning:-
Sam Alfaras = Horse stopped feeding (when used for horse) Sam Alma = Water stopped flowing (when used for water) Sam Alrih = Wind stopped blowing (when used for air) and Sam AnAlta’am = Man stopped eating (when used for food)
Like the word Nation (Qawm), the paradigm of soum is action (Fael) with the root letters saad, waaw, and meem (م و ص), as mentioned above. Other words derived from this root are Sam, Yasoum, Souma, and Siama and they means to stop whatever someone is doing. The Arabic dictionary Al Qamos at page 953 is defining Al soum as to stop the conversation/ speech or to stop your act/action
But in Islam of today, to stop talking or to stop action is supposed to mean "stop eating, drinking and having intimate relations with wife".
In this aya the PURPOSE of Soum is provided, to make you MUTTAQI. So Soum is a tool to become Muttaqi, what it is, is defined in 2:177
2:183 Oh Momins, Siam ,to stop perpetrating evil and do good, is made compulsory on you just like it was made compulsory for people before you, in order to make you cohesive with Ehkamate Elahi-