The meaning of seh
In Jamaican Patois at least, seh is a cognate of say.
For example, from JamaicanPatwah.com¹:
Patois: Wah yuh seh?
English: What are you saying?
and from The Poetry Archive:
In Creole, where a noun refers to a class of persons or things, as such, or where it is preceded by a number or some other expression of quantity, nothing is added to indicate plurality.
Dem seh / wi commandeer cyar
They say / said we commandeered cars
('Di Great Insohreckshan', Linton Kwesi Johnson)
So in your example, Mi know seh dem nuh like mi, I'd read this as I know they say they don't like me, rather than your reading of that.
Linguists have used the term "lexifier" for the relationship between English and Jamaican patois or creole: that is, English supplies most of the words to the patois. Then they're subject to the phonology of that dialect.
Patois does interesting things with word-ending vowels, but I'm not versed enough to name the specific phonological alterations that produce "seh" from "say"; sufficed to say, the source (etymology) of "seh" in patois is "say" in English.
But, just like any other word, seh has evolved organically over time, and diverged from straight English say. Below, I'll review a couple of these extensions.
In set phrases
In certain set phrases, it has a different meaning, but that's more a function of the idiomacity of the phrase (taken as a whole) as opposed to the word.
From TheCultureTrip.com, 15 Jamaican Patois Phrases To Know:
Weh yuh ah seh
Literally translated as what are you saying, but actually meaning how are you doing. For example: Weh yuh a seh? Mi deh try call yuh means, How are you doing? I’ve been trying to call you.
To make the difference between a standalone word and a set phrase clearer, consider an AmE example: you could say "bucket" has a different definition in the phrase "kick the bucket", but that's misleading: it's the set phrase which has a specific meaning, and should be treated as a indivisible word (technically a lexeme).
Here's a case where seh apparently has an entirely different meaning, and I don't know how to class it.
From Paxton Belcher-Timme, Rhetoric of Reggae, Patois: the language of Jamaica (December 1, 2009):
Patois: cooh deh, dem ah galang lakka seh dem nuh ha nutten
English: Look at that! They are behaving as if they do not have anything.
Cooh Deh! = Look at that!
Deh ah = They are
Galanga lakka she = Behaving as if
Here, note the example sentence has Galanga lakka seh, but the "English translation" has Galanga lakka she.
So I don't know:
- which is the typo (meaning, this might not be discussing the word seh at all, but rather she),
- or whether this is a set phrase,
- or an entirely different meaning for the standalone word seh.
- Though I find lakka seh for like saying a plausible semantic broadening to "acting like" (it's like they're saying for acting as if²).
Possibility as a function word
Janus Bahs Jacquet, one of our resident linguists, in a comment, made an interesting connection to generic complement markers in Japanese:
I have nothing really to back it up, but it seems likely to me that ‘say’ has been reinterpreted as a generic marker of certain types of complement clause in Jamaican.
It ‘feels’ similar to how the marker と to functions in Japanese: its primary use is to mark direct speech, but it’s also used for things that can sort of be loosely compared to direct speech, such as the objects of sense and impression verbs, loose comparisons (act like, be like, look like), etc.
Assuming that seh is the right word in the last example, I think that and the one in the question are actually the same usage.
I'm in no better position than Janus to corroborate this theory, but from the few examples I've examined today, it feels plausible.
¹ Patwah here being patois for Patois
² I didn't look this up, but if I had to guess, galanga is derived from "going along", "proceeding". Thus, galanga lakka say would be "going along like they're saying", i.e. proceeding as if. But again, this is speculation.