In A1, there is a long lesson that tries to tell us how to infer a word's gender based on word ending patterns. Based on that, I can correctly guess that both "lave" and "vaisselle" are feminine, so why does dictionaries tell me "lave-vaisselle" is masculine ?
Bonjour Joakim! Once again, a very interesting question! First of all, remember that this A1 lesson is a "guide" but unfortunately doesn't apply systematically. In the case of compound nouns (hyphenated), when the word is built on a "verb+noun" pattern like here ("lave-vaisselle" - dish washer), it's mostly masculine. I hope that's helpful!