Geeky is the adjective from geek. The older, unkind use is the one many learner dictionaries still put first: someone who is not popular, who wears unfashionable clothes, who does not quite know how to be in a group, or who does things other people find strange. That use is still alive, and it can hurt if you aim it at a person’s looks or social life. Though actually the word has shifted. From the 1990s on, people started wearing it with a smile, especially about computers, games, comics, or a very specific hobby. “That’s a bit geeky” can now mean “deeply into the details, and that’s all right.” Younger speakers often hear the kind use first. The mean use and the fond use live side by side, so tone does a lot of work. It is informal either way. It is not simply “clever”: a geeky person may be clever, but the flavour is uncool, or happily obsessive, not “good at school”.