Green’s Dictionary of Slang

gen n.2

also jen
[? RAF gen(eral information) for all ranks]

information, facts.

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[UK] ‘A Hudson Song’ in Ward-Jackson Airman’s Song Book (1945) 121: When we go out to Norway / We bring ’em back the gen [....] They know they’re lost but, dammit chaps, they also know their gen.
[Aus]Sydney Morn. Herald 11 Dec. 7/2: Accurate information is either ‘the griff,’ ‘the gen,’ or the ‘good oil’.
[US]E. Hemingway letter 14 Apr. in Baker Sel. Letters (1981) 585: So that’s the Gen as of now.
[UK]K. Amis letter 15 Dec. in Leader (2000) 23: I will then slip the gen across to you ole boy.
[WI]S. Selvon Lonely Londoners 165: The boys liming in Moses room, coming together [...] to find out the latest gen.
[UK]I. Fleming For Your Eyes Only (1962) 17: All the top gen.
[UK](con. 1940s) O. Manning Danger Tree 198: The gen is that the jerries are preparing a push on Alam Halfa.
[UK]S. Berkoff West in Decadence and Other Plays (1985) 92: Confess the utter wholesome gen [truth].
[WI]S. Selvon Eldorado West One 27: You will get all the gen at the exchange.
[Scot]I. Rankin Wolfman 36: Sometimes he gives me good gen.
[UK]P. Baker Blood Posse 177: Chico instructed Menzy to give him another ten bucks to keep the gen flowing.
[UK]T. Blacker Kill Your Darlings 88: I’ll give you the basic gen.
[Scot]L. McIlvanney All the Colours 164: ‘You get the gen, you pay for it’.
[UK]J. Meades Empty Wigs (t/s) 50: We had the house’s address. That was enough. That was the gen. That was the griff.

In phrases

gen out (v.)

(US) to figure out, to plan.

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[US]E. Hemingway in Baker Sel. Letters (1981) 25 Aug.: Can’t gen out yet exactly how or when to take Mary to Ketchum where she can convalesce.
gen up (v.)

(US) to provide someone with information, to learn a lot of information in a short space of time (usu. on a specific subject); thus genned up, well informed.

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[UK]C.H. Ward-Jackson It’s a Piece of Cake 31: Gen-up, to learn quickly, to swot — prior to a trade test board or some equally binding ordeal.
[UK]J.P. Carstairs Concrete Kimono 92: I spend hours in the Rue de la Paix genning-up.
[Scot]I. Welsh Decent Ride 134: Nane ay thum ken aboot the square sausage. Git fuckin genned up, ah’d say tae them landladies servin the brekkies at they B&Bs, the square fuckin sausage!