Wikipedia:

“Sterling later described it in an article originally published in SF Eye #5, in July 1989, as “a kind of writing which simply makes you feel very strange; the way that living in the twentieth century makes you feel, if you are a person of a certain sensibility.”

And:

“Slipstream fiction has been described as “the fiction of strangeness”,[4] or a form of writing that makes “the familiar strange or the strange familiar” through skepticism about elements of reality.[5] Illustrating this, prototypes of the style of slipstream are considered to exist in the stories of Franz Kafka and Jorge Luis Borges.[6]

h/t AS for pointing this one out.