Plumard is a colloquial French term for 'bed' or 'cot'. It's a charmingly informal word that adds a touch of everyday realism and intimacy to the lyrics.
In the song, Maurice Chevalier sings, "Mais dans un plumard, Ça n'a pas d'importance" (But in a bed, it doesn't matter). He uses this word to suggest that in the privacy of a bedroom, certain qualities like intelligence become less important than affection and connection, especially when one is young and in love. It's a word that evokes a sense of casual comfort and youthful romance.
Maurice Chevalier's "Valentine" is a charming and humorous look back at a man's first love. He remembers his "première maîtresse" [first mistress] with a full and intoxicating memory. They met on a rainy day, were immediately attracted to each other, and their affection grew. He asked her name, and she said: Valentine.
He describes Valentine with a playful focus on her small features: "tout petits petons" [tiny little feet], "tout petits tétons" [tiny little nipples] that he'd "tâtais à tâtons" [feel by touch], and a "tout petit menton" [tiny little chin]. He also mentions she was "frisée comme un mouton" [curly like a sheep] and not very smart, but he says that in bed, "Ça n'a pas d'importance" [That doesn't matter]. He was crazy about her despite her being jealous and bossy. The song takes a funny turn when he meets a large woman on the boulevard years later, who greets him with "Bonjour, mon loup" [Hello, my wolf]. He doesn't recognize her until she reveals she's Valentine, now with a "double menton" [double chin] and "triple poitrine" [triple chest]! He's filled with dread, thinking how much she's changed from the girl with the tiny feet and tiny everything.