The story
Keyif is Turkish, and dictionaries give it a whole spread of meanings: pleasure, delight, good spirits and simply mood. It came into Turkish through Ottoman Turkish from Arabic, and the same Arabic root travelled in several directions. Azerbaijani has kef, for well-being and pleasant idleness. English picked up kif by way of Moroccan Arabic, for cannabis and the haze it brings, and modern Turkish slang has that sense too.
In everyday Turkish, though, keyif is mostly about mood and the care of it. Something keyifli is pleasant to be around, and someone keyifsiz is out of sorts. Asking after a person's keyif is a way of asking how they are. A word that sums up the whole idea is çakırkeyif: tipsy, with the cheerfulness of a drink or two but nothing worse.
The part that is hard to put into English is that keyif can be arranged. You do not wait for it to arrive. You make room for it: a glass of tea, a seat facing the water, an empty hour. Contentment is the honest translation, but contentment sounds like something that happens to you. Keyif is closer to a small project.
Picture a Sunday morning on the Asian shore of Istanbul. A man sits at a café table with a newspaper he is not really reading. His tea glass has been refilled twice without his asking, and a ferry slides past. He is not waiting for anything. He is taking his keyif.