And lead raptivity captive

What do raptivity and "And lead raptivity captive" mean?

It's difficult to tell, but here's my best analysis. I think raptivity is used here as a synonym for raptness, an uncommon word meaning the quality of being rapt. I can't tell from the text who's speaking or who they're speaking to, but to lead captive or hold captive mean to hold tightly to something, as though it were your prisoner. Since the line reads:

And lead raptivity captive. Ready! Like a Finn at a fair.

I guess that the command is intended to provoke a state of attention, readiness, or preparedness. Like a Finn at a fair refers to the eagerness or what have you that a person (apparently a Finnish person) would feel at a fair.

@John-Purdy is probably right about the meaning of "raptivity". "Lead raptivity captive" will be a reference to "lead captivity captive" in Ephesians 4:8. (And don't ask me what that means - it could be discussed all day if there was a theology stackexchange!)

What it means in the context I have no idea - Finnegans Wake is notoriously complex with strange made up words, grammar, etc. used for artistic effect. It's the sort of thing most native English speakers would struggle with.