Deformation of креветка (Shrimp). Can be used as an ironical light insult (ты — криветко — you are one wretched shrimp), and also as a form of ironical self-praise (йа — криветко). The origin is unclear, usually it is linked to the bash.org.ru quote shown below (another quote implies that it would actually be a Russian answer to the English question “who is Lobstarr?” that would have been written on a university desk), but the word has gained an impressive popularity within a very short time.
Да… конечно лекция по физике у нашего препода довольно увлекательное и серьезное мероприятие… Но когда перед тобой на парте красуется надпись: «ЙА КРИВЕТКО!»…
(bash.org.ru, 02/2007)