A wank window is limited time frame in which an opportunistic married man is alone in his home and can therefore peruse 'performing art' videos on the home computer whilst digitally stimulating his phallus. Mission control usually grants wank windows when the wife has gone to Tesco with the kids to do the shopping / gone to see relatives / went out with her mates. Wank windows are known to be tenuous and unpredictable because the wife may return early and the act of self flagellation may have to be aborted.
Courier: I tried to deliver the parcel to number 8 this morning but the bloke wouldn't answer the door. I rang the bell repeatedly but he appeared to be glued to his computer in the front room, ignorant twat!
Boss: How'd he look?
Courier: Funnily enough he was sweating, red faced and seemed utterly focused elsewhere.
Boss: Son, a man's wank window is sacred. When you're married you'll understand.