I’m not at all convinced that River’s more unusual speech patterns are a likely consequence of her illness. She seems to be channelling Spock as often as she’s spouting nursery rhymes; for example, in her reaction to Simon’s birthday (105 “Out of Gas”):
“Day” is a vestigial mode of time measurement based on solar cycles. It’s not applicable.
I didn’t get you anything.
Nevertheless it works for her, and I was particularly struck by the poetry of her apple bits speech. So here’s the whole scene (109 “War Stories”):
INT. SERENITY - SIMON’S ROOM - CONTINUOUS
The door slides open and Simon enters, seeing River sitting on a bunk.
Whoa! [Mei-mei… - “little sister”…]
SIMON covers a shivering RIVER with a blanket.
How you doing?
I threw up.
I’m sorry. It’s a side effect. We
just have to find the right treatment
for you. How do you feel now?
Going… Going back like… apple bits,
coming back up. Chaos.
But you felt okay this morning?
Played with Kaylee.
Sun came out, and I walked on my feet
and heard with my ears. I ate the bits,
the bits stayed down, and I work. I
function like I’m a girl.
I hate it because I know it’ll go away.
The sun goes dark and chaos has come
again. Bits. Fluids. What am I?!
You are my beautiful sister.
I threw up on your bed.
Yep. Definitely my sister.
The Urban Dictionary does not really clarify “fuckity” as a stage direction…