stumble, slip (also written as traspié)
A walking step with a syncopated cross. Using two beats of music the dancer does step-cross-step beginning with either foot and moving in any direction.
(from batango.com (http://www.batango.com/Loomis.htm))
Dancers have come to use the term to describe cutting the beat, or stepping on the half-beat (...)
(from tangocanberra.asn.au (http://www.tangocanberra.asn.au/words.htm#Traspie))
When the dancer stresses the musical accent down to the ground, it is called Traspie. When the dancer stresses the musical accent up in the air, it is called Traspie "falcetti".
(from pascaleyluis.com (http://www.pascaleyluis.com/article39.html))
Using two beats to make a triple step, thus stepping on one beat, then on the half-beat and again on the next beat, is a traspié. It is related to what is called syncopation, syncopa: the stressing of a normally unstressed beat, making a syncopated rhythm.
from Tango-E-Vita (http://users.pandora.be/Tango-E-Vita/tango/walk.htm)