hålla tummarna betyder på engelska

cross one's fingers▼(do this or a similar gesture to wish for good luck)
verb

keep one's fingers crossed▼(hope for the best)
verb
[UK: kiːp wʌnz ˈfɪŋ.ɡəz krɒst] [US: ˈkiːp wʌnz ˈfɪŋ.ɡərz ˈkrɒst]