very tall person
Humorous, colloquial term likening a very tall person to a fire lookout tower (半鐘) that a thief (泥棒) might climb to steal the bell. Not a literal thief.
That guy who's like a fire-tower thief—maybe he's a basketball player.
のっぽ is a more common, neutral slang for a tall, lanky person, while 半鐘泥棒 is a vivid, humorous metaphor.
Compound of 半鐘 (fire bell/watchtower) and 泥棒 (thief). The image is of a thief climbing a tall tower to steal the bell, humorously applied to a very tall person.