“psso” or pass slipped stitch over, is a knitting maneuver used in decreases.
Decreases & Abbreviations
Passing a slipped stitch over is used in centered decreases (sk2p, sk3p, etc), or in skp (which is a left leaning decrease). It is either abbreviated as the “p” in “sk2p” or as the final phrase in “sl-k2tog-psso.”
First: slip the number of stitches specified (usually one or two stitches) from the left needle to the right needle.
Then: knit, or k2tog, or k3tog as specified.
Finally: with the tip of the left needle pick up the slipped stitch, pull over the remaining stitches, and drop off the needle.