"Bread and butter" is a
superstitious blessing
In religion, a blessing (also used to refer to bestowing of such) is the impartation of something with doctrines of grace, grace, Sacred, holiness, spiritual Redemption (theology), redemption, or Will of God, divine will.
Etymology and Germani ...
or charm, typically said by young couples or friends walking together when they are forced to separate by an obstacle, such as a pole or another person. By saying the phrase, the bad luck of letting something come between them is thought to be averted. Both walkers must say the phrase,
and if they do not do this, then a bitter quarrel is expected to occur. The concept derives from the difficulty of separating butter from bread once it has been spread – buttered bread cannot be "unbuttered".
[ Another phrase used in this way is " salt and pepper".]
References
{{Superstitions
Luck
English-language idioms