I have heard "seh" used in Jamaican English but I think it's probably used in other parts of the Caribbean too. I know that in many cases, it is simply the equivalent of standard English "say". However, it is also used in sentences like:
"Mi know seh dem nuh like mi", meaning "I know that they don't like me"
In this case, "seh" seems to be used as an equivalent of standard English "that", rather than "say". But I would like to know whether I'm right, whether it has any other uses, and when people might choose to use "seh" instead of "that".
I'd also be interested to know about the etymology of the word.
Hopefully there are some Caribbeans on here who can shed some light on it.
Best Answer
The meaning of seh
In Jamaican Patois at least, seh is a cognate of say.
For example, from JamaicanPatwah.com¹:
and from The Poetry Archive:
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.
Etymology
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:
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).
Other
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):
Here, note the example sentence has Galanga lakka seh, but the "English translation" has Galanga lakka she.
So I don't know:
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'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.