Archaic word for U K (4)

I believe the answer is:

thou

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'k' is the definition.
(synonyms)

'archaic word for u' is the wordplay.
'archaic word for' becomes 'tho' (I can't justify this - if you can you should give a lot more credence to this answer).
'tho'+'u'='THOU'

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(Other definitions for thou that I've seen before include "Second person in the Bible" , "Old English pronoun" , "Prominent pronoun of the King James Bible; the modern-day 'you'" , "You (archaic, but still used in prayers)" , "1,000 (abbr)" .)

I've seen this clue in The Independent.

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