It looks like a regular slur, but at an obtuse angle. This example is from Liszt's Mephisto Waltz No. 1, where it connects an F natural to an E. I have only seen this a couple of times, so I'm just kind of curious what it's supposed to mean.
It's an accent that applies to both notes. In the Peters edition (2007, ed. Leslie Howard), bars 748 and 752, a footnote makes this explicit:
Liszt's special accent requires a stress on all the notes under the symbol.
In recordings you can often hear the accent implemented as an (extremely) momentary ritardando, as well as the usual increase in loudness.
Although not applicable to the piano piece in the question (the use of which is in the accepted answer), but to clarify for people that may see a similar mark used in student pieces, who may otherwise be confused:
In some instructional method books and corresponding pieces, the mark is used to indicate a half step in a new scale or fingering position. Sometimes it is added as a "courtesy mark", as a reminder of the half step, or for clarity.
Here is an example of use from the "Muller Rusch Violin Method" violin book indicating a half step between E and F natural:
Here is another example from "A Tune A Day" violin book indicating that the finger position for C# and G notes are a half step apart from each other:
Rubanks Elementary Method uses what looks like a Piano pedal bracket to indicate the half step instead.