Where does “глаголъ” come from?

глаголъ (Church Slavonic) comes from Proto-Slavic golgolъ, from Proto-Slavic gol-.

Ancestry of “глаголъ”, step by step

Step	Language	Word	Meaning
1	Proto-Slavic	golgolъ	speech; word
2	Proto-Slavic	gol-	—