1. day in, day out - Adverb
2. day in, day out - Phrase
day in, day out
(idiomatic) Every day; daily; constantly or continuously (especially, of something that has become routine or monotonous).
Even if you like peanut butter sandwiches, eating the same sandwiches day in, day out will get old.
Doing the same thing day in, day out. It's just so boring. I like to jump from different things. Michelle Ryan