Fig.: it refers to something considered worthless, stupid, no good, or just a waste of time

To be honest with you, I think that famous writer's last book is just for the birds. It was so boring and meaningless that I gave it up halfway through

Ad essere sinceri, penso che l'ultimo libro di quel famoso scrittore non valga niente. Arrivato a metà ho smesso di leggerlo, tanto era noioso e insulso

Non-alcoholic beer? It's for the birds!

Birra analcolica? Da buttare!

Your excuse for being late yet again is absurd, strictly for the birds! Frankly, I don't believe a word of it

La scusa per il tuo ennesimo ritardo è assurda, per non dire ridicola! Non credo a una parola di quello che hai detto

Don't listen to that tacky colleague of yours. His opinion on your look is for the birds, considering that he knows nothing about fashion or good taste and doesn't understand a thing about it

Non ascoltare quello zotico del tuo collega. La sua opinione sul tuo look non vale nulla visto che di moda e buon gusto non ne sa e non ne capisce niente

The original phrase "that's shit for the birds", started in the U.S. Army in World War II, based on the observation of birds repeatedly pecking at horse manure in search of seeds inside it. That's why whatever "It's for the birds" is considered worthless or useless, as it alludes to horse manure.
Later on, the term "shit" was removed from the phrase as it was considered too vulgar for a broader and more general usage.