Mleczko is the diminutive form of mleko (milk) in Polish, literally meaning "little milk." It's a charming and somewhat unusual word to feature prominently in a song title and lyrics, making it memorable.
In the song, the ex-partner's new love interest "brings warm mleczko" to him, contrasting with the narrator's more rebellious attempts to win him over. This word evokes a sense of domesticity, care, and perhaps even a touch of infantilization, highlighting the new girlfriend's seemingly perfect, nurturing role compared to the narrator's past efforts.
Ever tried way too hard to get someone's attention, only to be left behind? That's the story of "MLECZKO"! The singer looks back on all the crazy things she did for a guy who used to call her his 'baby' and his 'everything'. From wearing skirts in the rain and skipping class to even getting seven tattoos, she did it all just for him. The title itself, "Mleczko," means 'little milk,' which sounds a bit like how you'd talk to a baby.
Now, she's watching from the sidelines as a new girl enjoys all the benefits. This new girl drives his car, makes his bed, and brings him that 'warm little milk.' The singer sarcastically calls this her 'reward' for being so naive. It’s a fun, upbeat song about the frustrating feeling of doing everything for someone who moves on without a second thought.