A person who studies too much, or is too deep in some narrow interest, and is a bit socially hopeless because of it. British slang, and quite dated now — it belongs to school playgrounds and early internet forums of the 1980s and 90s, and younger speakers today are more likely to say nerd or geek. It is an insult, but a small one, closer to teasing than real cruelty, and people sometimes call themselves a spod half-proudly. There is a verb too: to spod, meaning to sit hunched over a computer for hours doing something obscure. Worth knowing the drift here: nerd and geek have become almost compliments in English now, but spod never made that journey — it stayed mildly unkind, and it stayed British. Because it is so dated, using it will mark you as someone who learned the word from an old book.