I notice few colleagues use 'post' almost everywhere. Today one of them said 'lets meet up post noon' and I thought shouldn't it be 'afternoon'? I could be okay with 'post lunch' but somehow 'post noon' felt very awkward.
"post noon" and "afternoon" are not interchangeable because "afternoon" refers to a specific time period during a day while "noon" refers to a single, specific time.
- early morning
Saying "post noon" means "let us meet sometime after noon." This is very distinct from:
INCORRECT — Let us meet sometime afternoon.
This sentence is incorrect because "afternoon" is a period of time. "Noon" is the very specific time of 12pm. You could say:
Let us meet sometime after 3pm.
Let us meet post noon.
Let us meet post dinner.
Let us meet during the afternoon.
Let us meet in the afternoon.
Let us meet this afternoon.
Let us meet post 3pm.
Option (7), however, is nonstandard and I recommend avoiding it. It isn't ungrammatical but it still sounds strange.
The constructs "post lunch" and "post noon" are both incorrect. While "post" can act as a noun prefix in some cases, it does not stand alone as a synonym to "after."