Lappen

Literally a small piece of cloth or leather, a 'rag' or 'cloth'. Wiktionary explicitly lists a sense '[5] colloquial, coarse: an insulting expression in youth slang' (roughly 'loser', 'wimp') - the image of a limp, strengthless cloth transfers onto a spineless person. Not to be confused with 'Lappen' = driving licence, another colloquial meaning.