Second person's half-expressed ideas (4)
I believe the answer is:
'second person's' is the definition.
(historical second person pronoun)
I don't understand the remainder of the clue.
Can you help me to learn more?
(Other definitions for thou that I've seen before include "You (arch.)" , "You (formerly)" , "You, in olden times" , "Old-fashioned pronoun" , "Old English pronoun" .)