Principal English Translation: 

to rain gently for a long time (see Karttunen and Molina)

Orthographic Variants: 
Alonso de Molina: 

tlapayaui. (pret. otlapayauh.) llouer mansamente y sin cessar.
Alonso de Molina, Vocabulario en lengua castellana y mexicana y mexicana y castellana, 1571, part 2, Nahuatl to Spanish, f. 130v. col. 1. Thanks to Joe Campbell for providing the transcription.

Frances Karttunen: 

TLAPAYAHU(I) to rain gently for a long time / llover mansamente y sin cesar (M) [(1)Tp.234, (5)Xp.93]. X has a long vowel in the second syllable, where T has a short one. If the vowel were basically long, then this could be understood to contain *PĀ ‘water,’ retaining the initial P which has been lost in Ā-TL. See QUIYAHU(I), ĀYAHU(I)-TL.
Frances Karttunen, An Analytical Dictionary of Nahuatl (Norman: University of Oklahoma Press, 1992), 290.

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