Half Sigma has previously discussed blogger Peter's working theory on how some activities invariably come to be classified as nerdy while others do not. Concisely put, non-athletic activities not traditionally regarded as masculine that are primarily participated in by men are nerdy.
That strikes me as a pretty good description, but as I'll get to below, there is an element Half Sigma and Peter leave out. Athletic and non-athletic activities need to be looked at separately, because the sex ratio effects them in different ways. The more male-dominated an athletic activity is, the less nerdy it is. American football is among the least (if not the very least) nerdy sport in existence. Female participation in the game is accordingly paltry. This is in some contrast to baseball, a relatively nerdier sport with a significant amount of female participation in the form of softball. Tennis, a sport in which female competitions garner something close to as much media attention as male competitions do, is nerdier still. Again, this nerdiness is relative to other athletic activities--by virtue of gauging some combination of dexterity, strength, physical endurance, and kinesthetic coordination (often referred to as "athleticism" in aggregate), athletic activities are virtually all non-nerdy.
In contrast, the greater the female participation in non-athletic activities is, the lower the level of perceived nerdiness among those partaking. Activities primarily participated in by women, like interior decorating, are not considered nerdy. If a man happens to take interest in them, he is often suspected of being gay or at least effete, but not nerdy. Nerdiness is the male's domain. To the extent that women can be nerds, it is in being exceptions to the rule and participating in male-dominated nerdy activities.
Where Peter is a bit off the mark is in focusing on the level of masculinity (in the sense of majority-male participation, not necessarily the amount of virility required) traditionally associated with an activity. As Half Sigma points out, if time is what's required to move a male-dominated non-athletic activity from the nerdy category into non-nerdiness, we've had to wait a long time for chess to come around, and I still don't see the train coming into the station.
A more useful parameter for gauging nerdiness among male-dominated activities is to consider the intelligence threshold required of participants engaging in them. After all, the commonly understood definition* of a nerd is "an intelligent but single-minded person obsessed with a nonsocial hobby or pursuit".
Chess is nerdier than checkers is because it requires greater intelligence to fully comprehend the elements of the game than checkers does. IT guys are similarly nerdier than auto mechanics are. The same goes for classical music votaries in relation to those who like rap. That is not to say that intelligence is unhelpful in becoming a champion checkers player, a top-tier car mechanic, or a hip hop connoisseur, but the participant pool of these activities extends further into the left half of the bell curve than it does for chess players, ITers, and classical fans. By way of being brainiacs, those who participate in nerdy activities engage in and discuss it with other brainiacs. Because they're all cerebral types, the level of discourse is such that average folks are unable to follow what the participants are talking about and consequently are also unable to see how it could be enjoyable and fulfilling from a 'normal' person's perspective (that is, their own).
That brings me to my own recently rekindled nerdy passion, Magic: The Gathering. Stripped of its Tolkienesque themes (with several historical references thrown in, many of them delightfully politically incorrect), which are completely irrelevant to actual gameplay, Magic is a competitive card game. It is to poker what chess is to checkers, except the gap is an order of magnitude wider than it is between the two board games. As the official rulebook demonstrates, the game's complexity is intimidating (and also rewarding). Most people can learn to play Texas Hold 'Em in a matter of minutes. Magic, in contrast, takes a few hours just to get the basics, and many people are going to be lost if attempting to go beyond that. A hold'em conversation is consequently comprehendible to most people, while overhearing a Magic conversation is like eavesdropping on a couple of klingons. With the essential aspects of a nerdy activity in place--male-dominated, non-athletic, high IQ threshold--Magic is quintessentially nerdy.
* Like the words "peruse", "terrific", and "awful", "nerd" is often used in a manner at odds with its primary literal meaning, which describes a foolish, inept, and unattractive person.