Green’s Dictionary of Slang

(orig. UK juv.) an earnest, moralizing lecture, esp. as delivered by parents or teachers; occas. as v.

1891		R.G.K. Wrench Winchester Word-bk 31: He pi-jawed me for thoking.
1913		A. Lunn Harrovians 211: I’ve just had an awful pi-jaw from my tutor.
1915		R.P. Hamilton diary 28 Jan. 🌐 Inspected billet. Turfed off by C.O. for being late and piffle jawed because I stopped him using accessories I think.
1923		O. Onions Peace in Our Time 203: And did you give us all this pi-jaw about our morals during the war?
1929		Kipling ‘Satisfaction of a Gentleman’ in Complete Stalky & Co. (1987) 254: ‘And giving ’em a pi-jaw?’ ‘Pi-jaws aren’t my line. There was a jaw, though.’.
1946		(con. 1912) B. Marshall George Brown’s Schooldays 142: You would have laughed your head off at the pi-jaw the Old Man gave us the other day.
1958		Willans & Searle Complete Molesworth (1985) 14: When getting pi-jaw about why you are being kaned.
1968		J.R. Ackerley My Father and Myself 79: He invited the two of us into the billiard-room [...] for a ‘jaw’, which could hardly be called ‘pi’ and which he himself described as ‘man to man’.
1980		(con. 1940s) O. Manning Sum of Things 459: Just like a school pi-jaw.