preachy priest; tub-thumping preacher; pulpiteer
Derogatory term for a Buddhist priest or preacher who is overly fond of lecturing or moralizing, often with a condescending tone. The word 坊主 (ぼうず) itself can be mildly derogatory when referring to a priest.
That preachy priest's talks are always long and preachy.
He points out people's faults like a pulpiteer.
説教 is a neutral term for a sermon or preaching, while 説教坊主 is a derogatory term for a person who preaches in an annoying or condescending way.
坊主 is a general term for a Buddhist priest, but can be used derogatorily; 説教坊主 specifically criticizes the priest's preaching style.
Compound of 説教 (せっきょう, 'sermon, preaching') and 坊主 (ぼうず, 'Buddhist priest'). The derogatory nuance comes from the combination of the critical view of excessive preaching and the sometimes pejorative use of 坊主.