John C. Bogle founded Vanguard, which had the first index funds. Simply put, "Boglehead" refers to someone who invests in index fund(s) and leaves the money there long-term, without trying to time the market, buy individual stocks, etc.

Thanks. My search-fu was not caffeinated enough this morning.

There’s an entire forum dedicated to this at https://www.bogleheads.org/.