Avoir beurre sur la tête. To be covered with crimes. Taken from a Jewish saying, “If you have butter on your head (i.e. have stolen butter and put it in your cap), don't go into the sun.” (Vidocq: Voleurs, vol. i. p. 16.)
J'y suis pour mon beurre. Here beurre means argent: I paid for it through the nose. Beurre or butter has the same relation to food as wealth has to civil life; it does not take the place of it, and does not make it, but it makes it go down more pleasantly, and adds somewhat to its wholesomeness. As Shakespeare says, “Where virtue is, it makes more virtuous.”
Promettre plus de beurre que de pain. To promise much, but perform little. To promise more than one, can, or chooses to, perform. The butter of a promise is of no use without substantial bread. “Be thou fed” will not fill an empty stomach. A little help is worth a deal of pity.
Source: Dictionary of Phrase and Fable, E. Cobham Brewer, 1894