tzipitl.

Headword: 
tzipitl.
Principal English Translation: 

a child who is unwell on account of expecting a sibling (see Molina)

IPAspelling: 
tsiːpitɬ
Alonso de Molina: 

tzipitl. la criatura que esta enferma o desgañada a causa de estar su madre preñada.
Alonso de Molina, Vocabulario en lengua castellana y mexicana y mexicana y castellana, 1571, part 2, Nahuatl to Spanish, f. 152v. col. 2. Thanks to Joe Campbell for providing the transcription.

Frances Karttunen: 

TZĪPI-TL pl: TZĪTZĪPIMEH the child who is youngest when a new sibling is born / la criatura que está enferma o desgañada a causa de estar su madre preñada (M), el niño que es más chico cuando nace otro (T) [(1)Tp.243].
Frances Karttunen, An Analytical Dictionary of Nahuatl (Norman: University of Oklahoma Press, 1992), 315.

Attestations from sources in English: 

chipil = a Hispanization of tzipitl, known in Mexican Spanish today
David O. Bowles, post in X (then Twitter, social media), June 18, 2021, https://x.com/davidobowles/status/1405943055350390791?prefetchTimestamp=...

themes: 
Children, Childhood, Adolescence