Where does “prostořečný” come from?

prostořečný (Czech) comes from Czech řeč, from Old Czech řěč, from Proto-Slavic rěčь, from Proto-Slavic rěk-, from Proto-Slavic -ь, from Proto-Indo-European -is — Forms nouns from verbs.

Ancestry of “prostořečný”, step by step

prostořečný traces back through two separate lines of ancestry.

via Czech řeč

StepLanguageWordMeaning
1Czechřečspeech; speech, oration; language
2Old Czechřěčspeech
3Proto-Slavicrěčьspeech
4Proto-Slavicrěk-
5Proto-SlavicForms nouns; From adjectives; From participles
6Proto-Indo-European-isForms nouns from verbs

via Czech prostý

StepLanguageWordMeaning
1Czechprostýsimple; free of; injective
2Proto-Slavicprostъsimple, straight
3Proto-Indo-Europeanpro-sth₂o-
Every word from Proto-Indo-European -isEvery word from Proto-Slavic Every word from Proto-Slavic rěk-