to wash one's head (reflexive) or to wash someone's head (transitive)(see Molina)
Orthographic Variants:
quapaca
Alonso de Molina:
quapaca. nino. (pret. oninoquapac vel. oninoquapacac.) lauarse la cabeza.
quapaca. nite. (pret. onite quapac.l. onitequapacac.) lauar a otro la cabeza. Alonso de Molina, Vocabulario en lengua castellana y mexicana y mexicana y castellana, 1571, part 2, Nahuatl to Spanish, f. 85r. col. 1. Thanks to Joe Campbell for providing the transcription.