Babies spit up two kinds of milk: one that's still fresh, and one that's been in the stomach long enough to look like cottage cheese. As soon as we learned this, Jack and I began saying, "I've been cheesed!" when the baby spit up on us. As cheese went from noun to adjective to verb in our house, all sorts of phrases sprung up, such as "Don't jiggle Jackson too much or he'll cheese you!" and "You're looking particularly cheesy this evening" and "Hand me that cloth, I have to decheesify someone here." And of course it led to an evolving series of nicknames for the baby (Mr. Cheese, Monsieur Fromage, Signor Formaggio) leading to the current favorite: Senor Queso.
His other nickname is The Nut, which stems from the beginning of my pregnancy when I started consulting a little growth chart to see how much the fetus weighed each week. Somewhere around 9 weeks I determined that s/he was as big as a peanut, so that's what Jack and I started calling the little beast (handy when we were still debating names). For a long time s/he was going to be named Pablo Ali (or Kate), but serious dissension from Jack's mom led us to settle on Jackson.