köken: Inherited from Old Spanish cielo, from Latin caelum, from Proto-Italic *kailom, from Proto-Indo-European *keh₂i-lom (“whole”), from *keh₂i-.
aile: a cielo abierto, a cielo descubierto, árbol del cielo, arco del cielo, azul cielo, caído del cielo, cielo de la boca, cielo raso, clamar al cielo, como caído del cielo
köken: Inherited from Old Galician-Portuguese ceo (“sky; heaven”), 13th century, Cantigas de Santa Maria; from Latin caelum (“sky”). Cognate with Portuguese céu and Spanish cielo.
aile: ceo da boca
1326, López Ferreiro, Antonio (ed.): Galicia Histórica. Colección diplomática. Santiago: Tipografía Galaica, page 294
1326, López Ferreiro, Antonio (ed.): Galicia Histórica. Colección diplomática. Santiago: Tipografía Galaica, page 295
From Middle Dutch hêmelen. The word is probably related to heimelijk (“secret”), and in early modern Dutch it still meant "to clean up, to tidy". Semantic influence from hemel (“heaven”) likely shifted its meaning from this towards "to make nice, to make beautiful" and then to the current sense.