....Okay then. Taking Estha's response at face value: yes, the thing she said was really stupid, just, in a different and even worse way than what she'd been worried about!
Wow! You know what would be really nice: if Merrin could actually! trust! that this was more-or-less Estha as she knows him - just with additional bonus [spells] from a [god], somehow - and therefore trust him to be acting with full cooperation and telling the truth when he says that this topic is genuinely actually cognitohazardous and that is definitely not a self-serving justification for scaring her into not asking any more questions!
"Understood," Merrin says immediately, and - she wasn't really thinking ""about"" the...things that she is now also not thinking about the vocabulary term for...due to literally not knowing anything more than the connotations she could pull out from introspection, but she can now can continue not-thinking about that whole vaguely-defined corner of conceptspace in her new linguistic vocabulary, she does have that much mental discipline. Estha did seem actually stressed, she thinks? But, of course, she's not sure that she can read him better than he can conceal things from her.
Anyway. Not knowing Estha's deal is presenting Merrin with the dilemma that, on the one hand, maybe it is UNWISE to be trying to extract information from linguistic introspection on her own and she should STOP and just learn what he judges is a good idea to tell her. But ALSO this is one of the few ways she can learn anything about him from sources that are even slightly uncorrelated with "exactly what he decides he wants her to know", and, just, arghhh.
(...Speaking of vocabulary and concepts, what was that word he said? [Soul], that one, it was the one that most obtrusively lacked an immediately obvious 1:1 Baseline transaction...?)