Where does “мечта́тельный” come from?

мечта́тельный (Russian) comes from Russian мечта́ть, from Old Church Slavonic мьчьтати, from Proto-Slavic mьčьtati.

Ancestry of “мечта́тельный”, step by step

Step	Language	Word	Meaning
1	Russian	мечта́ть	—
2	Old Church Slavonic	мьчьтати	—
3	Proto-Slavic	mьčьtati	—

мечта́тельный — Etymology, Origin & Word History | Etymology Explorer