Principal English Translation: 

something small; perhaps an insect, like an ant?

Attestations from sources in English: 

The Codex Mendoza includes two examples of the place name Yacapichtlan, and they both have a nose (yacatl) and an insect that looks like an ant that could be a pichin. The Matrícula de Huexotzinco (folio 615 verso) includes Cuapich as the name of a man, and the glyph looks like an ant.
See, for example, https://aztecglyphs.wired-humanities.org/content/yacapichtlan-mdz24v .

Attestations from sources in Spanish: 

pichintzin = un poquito
David Harold Tuggy, Lecciones para un curso del náhuatl moderno, 1991, 120.