The term "bar" first appears in Lacan's work in 1957, where it is introduced in the context of a discussion of Ferdinand de Saussure's concept of the sign.In this context, the bar is the line that separates the signifier from the signified (in the Saussurean algorithm), and stands for the resistance inherent in signification which is only crossed in metaphor.
The Barred Subject
Not long after the 1957 paper in which the term first appears, in the seminar of 1957-8, Lacan goes on to use the bar to strike through his algebraic symbols S and A in a manner reminiscent of Heidegger's practice of crossing out the word "being."The bar is used to strike through the S to produce, StrikeS.gif, the "barred subject'." The bar here represents the division of the subject by language, the split. Thus whereas before 1957 S designates the subject (e.g. in schema L), from 1957 on S designates the signifier and StrikeS.gif designates the (divided) subject.
The Barred Other
The bar is also used to strike through the A (the big Other) to produce the algebraic notation for the "barred Other," A. However, Lacan continues to use both signs in his algebra (e.g. in the graph of desire). The barred Other is the Other insofar as it is castrated, incomplete, marked by a lack, as opposed to the complete, consistent, uncastrated Other, an un-barred A, which does not exist.