I like hearing some Sheng here in the comments section! It's great when people add in from different dialects
Well it's coming from English slang 'I feel you' meaning I understand you, or I agree. It's common in Swahili to take English verbs and slap a na- and an object prefix and pretending its a Swahili word.
Yeah. I like that. It makes it easier to fake it if you don't remember a word. But there are also a lot of other Sheng words that doesn't come from English, even though the word Sheng is a portmanteau of Swahili and English. Often you have nonstandard ways of using Swahili words, like for example "piga stori" - "beat a story", meaning "tell me something good". Or another one "msee" (which I believe is a twist on the word "mzee"), used for addressing someone you don't know, almost like "hey, mister".
Piga story is actually the perfect example of using English words in Swahili. You may know how versatile the word piga is from the "piga" skill. It is used with many words, where the meaning of piga is not literally "beat," such as piga gita- to play guitar(or substitute any instrument) piga picha- to take a picture, piga magoti- kneel down, etc. So this is just taking the versatility of piga and using an English word with it to express the thought "telling story(ies)." But yes, it doesn't have to be English words, which are used in Swahili, it can be others, but since basic English, as well as some English slang, is common among Waswahili it is most often. For example, one of my neighbors once said "alinisema shiti"- She was talking ❤❤❤❤ to me.
As for Msee it is actually the same as Mzee. It sounds like you are in a Swahili country now, so if you are there for a while you may eventually see how flexible Swahili is in spelling and pronunciation. Particularly the letters 'l'and 'r' are pronounced almost exactly the same and to a lesser extent so are 'z' and 's.' Because of this, while there are 'official' spellings, words can be spelled with l or r, or z or s, interchangably, among other changes that you will see in written Swahili. This is because many people in countries like Tanzania and Kenya and Uganda etc. still do not know how to read and write properly, so they are even more impacted by Slang language than people in America or Europe.
Also while Mzee literally means old (man), it is used interchangably with "mister." You may have already heard Waswahili shout at you "MISTA, MISTA!"