Green’s Dictionary of Slang

1. (UK school, also sapper) a hard worker.

1798		C. Smith Young Philosopher I 48: He obtained the character of a sullen, cold-blooded fellow, and a sap [OED].
1828		Lytton Pelham I 11: When I once attempted to read Pope’s poems out of school hours, I was laughed at, and called ‘a sap.’.
1843		R.S. Surtees Handley Cross (1854) 168: Saps come at opening, others at noon, honourables not till afternoon.
1850		F.E. Smedley Frank Fairlegh 117: They pronounced me an incorrigible sap.
1864		B. Hemyng Eton School Days 68: You know I hate saps.
1888		Times 1 Feb. 12/2: Remember the many epithets applied to those who, not content with doing their work, commit the heinous offence of being absorbed in it. Every school, every college has had its choice nickname for this unfortunate class... such as a ‘sap’, a ‘smug’, a ‘swot’, a ‘bloke’, a ‘mugster’.
1898		Boy’s Own Paper 19 Nov. 117: One fellow, who had the character of being a quiet and unobtrusive ‘sapper,’ and who was invariably down at his books half an hour before the other fellows.
1899		‘Pot’ & ‘Swears’ Scarlet City 82: I [...] managed to get the better in ‘Trials’ and ‘Collections’ over the most hard-working ‘saps’ in my Division.
1914		‘Ian Hay’ Lighter Side of School Life 105: A great company of which the House recks nothing [...] the Cave-Dwellers, the Swots, the Smugs, the Saps.
1922		(con. 1900s) S. Leslie Oppidan 48: You must be an awful sap.
1953		R. Bissell A Gross of Pyjamas (1954) 9: He had a sap called Mr. Myron Hasler who did all his dirty work for him.
1959		I. & P. Opie Lore and Lang. of Schoolchildren (1977) 201: The word ‘sap’, which at Eton is primarily used to castigate someone who is over-keen on his work.

2. a derog. term for an intellectual.

1850		F.E. Smedley Frank Fairlegh (1878) 126: They pronounced me an incorrigible ‘sap’.
1879		‘’Arry on the ’Igher Education of Women’ in Punch 5 Apr. in P. Marks (2006) 151: Mathematticks and Jography’s rot he may leave to the Sap and the Swell.
1890		‘’Arry on [...] the Glorious Twelfth’ in Punch 30 Aug. 97/2: ‘Loves the Moors much too well for to carry one;’ that’s wot he says, sour old sap.