Savey (savvy) is a common vulgarism for knowing, knowledge; as verb: to know, to understand.
Do you savey? = Do you know, do you understand? He had plenty of savey = of savoir faire, or savoir vivre.
»Upon my sivvy» stands probably for »upon my affy», a common vulgarized form of »upon my solemn affidavit» (upon my sworn testimony.)
The term is no doubt derived from some Romanic language, perhaps a corruption of the French vous savez or savez-vous.
Sawny-headed
»Well, you was a sawny-headed chunk, Josiah, you was.» (Novel Notes 36. 15.)
= a blockhead, a simpleton.
Sawny is said to be a corruption of »Sandy», the usual nickname (abbrev. of Alexander) for a Scotsman, as Taffy (Welsh pronunciation of Davy) for a Welshman, and Paddy (short for Patrick) for an Irishman.
Chunk = a shapeless mass of anything, a thick piece, a lump: of wood, bread, etc.; as applied to a person = blockhead. (Cf. chump.)