"Bread and butter" is a superstitious blessing 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. ^[1] Both walkers must say the phrase, ^[2] and if they do not do this, then a bitter quarrel is expected to occur. ^[3] The concept derives from the difficulty of separating butter from bread once it has been spread – buttered bread cannot be "unbuttered". ^{[2] [4]} Another phrase used in this way is "salt and pepper". ^[5]

References

Look up bread-and-butter in Wiktionary, the free dictionary.